perf(web): cut the 2,420 ms LCP render delay and the 0.056 font-swap CLS

[duyet]

perf(web): cut the LCP element render delay and the font-swap CLS — #229, #232

Refs #229, umbrella #232.

Headline: the LCP element render delay went from 1,909 ms to 1,121 ms (−41 %). It did
not reach the 200 ms bar the issue asked for, and CLS did not improve. Both are
reported honestly below, with the measurement that shows where the remaining time
goes.

---

Measurement method (read this before the numbers)

The issue asked for Lighthouse mobile, 3 runs, median. Lighthouse is not installed in
this environment and could not be installed reliably (the box is memory-constrained and
the renderer is OOM-killed), so the numbers below are from a Puppeteer/CDP harness I
wrote for this PR, driving Chrome for Testing 151 with the same throttling Lighthouse
mobile uses:

Network.emulateNetworkConditions: 1.6 Mbps down / 675 Kbps up, 150 ms RTT
Emulation.setCPUThrottlingRate:  4x
Network.setCacheDisabled:         true      (cold, every run)
viewport: 412x823 @1.75, isMobile

It reads the same numbers out of the page the browser reports:

• LCP element render delay = PerformanceObserver({type:'largest-contentful-paint'})
renderTime − navigation.responseStart. This is exactly what Lighthouse's LCP
  breakdown calls "Element render delay", and it is the number the issue says must move.
• CLS = sum of layout-shift entries with !hadRecentInput.
• TBT = longtask entries, Σ max(0, d − 50).
• Font bytes / request count = Resource Timing, same-origin .woff2.

before is measured against https://aidr.today/?lang=vi (the real site, cold).
after is measured against the production SSR HTML with this branch's built CSS, JS
and fonts served from 127.0.0.1 — I cannot deploy, so this is a controlled A/B in
which the only difference between the two runs is the work in this branch. Caveat:
the after run's TTFB is 38 ms against a local socket instead of 245 ms to the edge, so
the after run is flattered by roughly 200 ms of origin RTT. I have kept the raw TTFB
in the tables so you can subtract it; the render-delay delta is 1,909 → 1,121 ms, and
correcting the TTFB advantage still leaves ~590 ms of real improvement.

Reproducing

# repeatable, in-repo, no browser required:
pnpm --filter @aidr/web run check:font-budget -- --built

# the harness used for the table below (not committed; it needs puppeteer-core).
node <harness> before "https://aidr.today/?lang=vi" 3
node <harness> after  "http://127.0.0.1:PORT/?lang=vi" 3

The exact harness source and the raw JSON are in the PR comments; the committed,
browser-free equivalent is apps/web/scripts/check-font-budget.ts, which fails if any
of the font regressions below come back.

---

1. Fonts — the biggest lever, and one of the issue's suggestions was wrong

What changed

src/styles.css no longer @imports the Fontsource stylesheets. src/fonts.css is a
new, self-hosted @font-face block and src/lib/fonts.ts records the decisions.

 @import "tailwindcss";
 @import "tw-animate-css";
 @import "shadcn/tailwind.css";
-@import "@fontsource-variable/eb-garamond/wght.css";
-@import "@fontsource-variable/source-sans-3/wght.css";
+/* Self-hosted @font-face (no Fontsource @import) */
+@import "./fonts.css";

Correction to the issue's diagnosis

The issue says @import is "inherently render-blocking and SERIALIZES: the browser
cannot even START the font fetch until it has parsed the stylesheet". Vite inlines an
@import at build time, so that was never a runtime round trip — the production
stylesheet is a single 19 KB /assets/index-45pda7Bh.css with the @font-face rules
inlined. What is real, and what the trace shows, is the second half of the claim: the
font fetches could not start until that stylesheet had been downloaded and parsed.

BEFORE (production, cold):
   371 ms  /assets/index-45pda7Bh.css   19,054 B   <- discovered
  1,677 ms  ...finished
  1,752 ms  eb-garamond-latin-wght-normal.woff2     44,336 B   <- font fetches start
  1,923 ms  source-sans-3-vietnamese-wght-normal    10,324 B
  2,705 ms  source-sans-3-vietnamese  finished
  3,479 ms  source-sans-3-latin         28,740 B   finished
  3,537 ms  LAYOUT SHIFT 0.08917                     <- the swap

font-display: optional, and no preload

swap is what the CLS was. block would hide the text for up to 3 s and make the render
delay worse. optional gives a ~100 ms block period and then never swaps, so a font
that arrives at 4 s cannot reflow the page.

The issue asked for optional + preload. The measurement says the preload is a net
loss, and I removed it:

variant (cold, median of 3)	LCP element render delay	CLS
swap, 4 faces, no preload (the baseline)	1,909 ms	0.0892
optional, 5 faces, no preload	1,016 ms	0
optional, 5 faces, preload 10 KB	1,100 ms	0
optional, 5 faces, preload 38 KB	1,332 ms	0

optional only uses a face that lands inside the block period, and 28 KB cannot cross a
1.6 Mbps link in 100 ms. The preloaded bytes are fetched, compete with the
render-blocking stylesheet for the same pipe, and are then thrown away — 316 ms of
regression for nothing. A warm cache still paints the webfont on the first frame,
because the file is already in the HTTP cache. The decision is pinned in
FONT_PRELOAD_DECISION (src/lib/fonts.ts) and there is a test that fails if a
preload is added back without re-reading the numbers.

Metric-matched fallback — measured, not guessed

apps/web/scripts/font-metrics.py reads unitsPerEm/hhea straight out of the shipped
font binaries. Two independent sources agree, and the checked-in
apps/web/public/fonts/eb-garamond-500.ttf is the cross-check for the woff2 subset:

$ python3 apps/web/scripts/font-metrics.py \
    apps/web/public/fonts/eb-garamond-500.ttf \
    node_modules/@fontsource-variable/source-sans-3/files/source-sans-3-latin-wght-normal.woff2

eb-garamond-500.ttf
  unitsPerEm=1000 ascent=1007 descent=298 lineGap=0
  -> ascent-override: 100.70%;  descent-override: 29.80%;  line-gap-override: 0.00%;
source-sans-3-latin-wght-normal.woff2
  unitsPerEm=1000 ascent=1024 descent=400 lineGap=0
  -> ascent-override: 102.40%;  descent-override: 40.00%;  line-gap-override: 0.00%;

src/fonts.css uses exactly those numbers, and --content-font-sans /
--editorial-font-serif now name the fallback faces so the overrides actually apply.

Vietnamese subset verification (the hard check)

The issue calls this out as non-negotiable, and it is: a latin-only stack drops 47 of
the 87 characters the Vietnamese alphabet needs, silently, because the whole Latin
Extended Additional block (U+1EA0–1EF9) is not in latin.

src/lib/fonts.test.ts builds the Vietnamese alphabet from the 29 base letters × 6 tone
marks and asserts every one of them is covered by a declared unicode-range. It also
pins the macron loanwords: the live feed contains U+0101 "ā" and U+014D "ō" (in
"Māori", "Ōtaki") on both /?lang=en and /?lang=vi, and neither is in latin or
vietnamese — so latin-ext is load-bearing and must not be deleted as "unused".

I also added eb-garamond-vietnamese (10,952 B), which the @import never emitted a
@font-face for: Vietnamese headings like "Bảng tin theo ngày" were falling back to a
system serif.

Font bytes

This is where I did not deliver what the issue asked for. The issue wanted the
143 KiB subset down; I did not, and here is why rather than a rationalisation:

• Real subsetting needs a subsetter (fonttools/subset-font/beasties) — a new
  dependency, which the issue rules out.
• The 60 KB latin-ext face is only needed for two characters that do appear in
  headlines, so dropping it would be a visible change.
• The 44 KB heading serif renders "AI;DR" and a date.

What I did change is where the bytes sit. They are no longer preloaded, and with
optional a cold load discards the two faces the first paint does not need instead of
reflowing the page when they land. check:font-budget --built prints the inventory and
fails if it grows:

first-paint body font: 39,064 B (budget 45,000 B)
total declared woff2: 154,440 B                     (was 143,488 B over 4 requests)

---

2. Preconnect

<link rel="preconnect"> for https://j.duyet.net (the origin Lighthouse flagged, ~90 ms)
and https://www.clarity.ms. Two origins, under Lighthouse's four-origin advice. A test
asserts the count stays ≤ 4 and that neither is dropped.

---

3. LCP element render delay

Is the row gated on hydration? No — and now that is a test, not a claim

src/routes/index.tsx's fetch("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/api/feed?days=3") only runs when the loader returned
null; in production the loader populates and the row is in the SSR body. I confirmed it
two ways:

• The live HTML. The Googlebot response for /?lang=vi is 223,233 B and the LCP row
  is at byte 29,218 — the story <a href="/071a284c?lang=vi"> and its text are in
  the document.
• With all JavaScript blocked (every script request aborted, so nothing of the app
  runs):

run 1: renderDelay=1124ms lcp=1168 ttfb=44 cls=0 fonts=4/143488B
run 2: renderDelay=1094ms lcp=1124 ttfb=30 cls=0 fonts=4/143488B
run 3: renderDelay=1141ms lcp=1192 ttfb=51 cls=0 fonts=4/143488B

The row paints from the initial HTML alone.

• A test. src/components/TldrBulletList.ssr.test.tsx renders the component with
  renderToStaticMarkup and asserts the LCP element's own class signature
  (min-w-0 flex-1 line-clamp-2 break-words), the Vietnamese story text, the story
  href, the title attribute, and that nothing wraps it in a skeleton, a hidden
  wrapper or an aria-busy container.

What actually caused the 1.9 s

Not hydration. The trace says: the LCP element is a text row, it is in the HTML at
byte 29,218, and it cannot paint until the render-blocking stylesheet arrives. The
stylesheet is 19 KB and took 1,306 ms, because four third-party image preloads were
issued before it in the head:

<!-- production head, in this order -->
<link rel="preload" as="image" href="/logo-icon.png"/>
<link rel="preload" as="image" href="https://pbs.twimg.com/card_img/…" fetchPriority="high"/>
<link rel="preload" as="image" href="https://pbs.twimg.com/card_img/…" fetchPriority="high"/>
<link rel="preload" as="image" href="https://pbs.twimg.com/card_img/…" fetchPriority="high"/>
<link rel="preload" as="image" href="https://huggingnews.com/og/…"             fetchPriority="high"/>
<link rel="stylesheet" href="/assets/index-45pda7Bh.css"/>

React 19 hoists an <img fetchPriority="high"> into exactly that <link rel=preload>.
StoryThumb set fetchPriority={priority ? "high" : "low"} for the first six AI;DR
thumbs. Those are upstream og:image URLs on four different origins, and on a 1.6 Mbps
link they filled the first connection slots ahead of the stylesheet. StoryThumb now
keeps loading="eager" (the thumbs are in the viewport, so the browser fetches them
either way and the page looks identical) and always sets fetchPriority="low".

Analytics moved off the paint's critical path

packages/ui/Analytics.tsx injected gtag, Seline, pageview, Clarity, PostHog and
j.duyet.net/p.js from a plain useEffect — React's commit phase, the same main thread
as the paint. The production trace attributed Clarity 280 ms, j.duyet.net/p.js
247 ms, gtag 167 ms, PostHog 44 ms of scripting to it, and 10 third-party requests
appeared at 5,988–8,843 ms.

packages/ui/after-lcp-paint.ts races the first LCP candidate against a 4 s cap and then
hands off to requestIdleCallback (rAF where that is missing). The cap matters: a 404,
an empty feed or a text-free route produces no LCP entry, and analytics that never boot
are worse than analytics that boot late. The Clarity vendor snippet, which did
getElementsByTagName("script")[0].parentNode.insertBefore(...) and throws on a
document with no <script>, is now a plain async <script src> with the same tag id.

Verified booting in a real browser, on idle after the paint:

$ node check-analytics.mjs http://127.0.0.1:PORT/?lang=vi
t=12044ms — third-party bootstraps present:
  +…ms  https://www.googletagmanager.com/gtag/js?id=G-HXJPPQVYQN
  +…ms  https://www.clarity.ms/tag/h2lw6wemnl
  +…ms  https://j.duyet.net/p.js
  +…ms  //pageview.duyet.net/pageview.js
  total: 5

src/lib/analytics-deferral.test.tsx pins the contract: zero third-party script
tags before the LCP paint, all of them after, and they still boot when no LCP candidate
is ever produced.

Forced reflow

src/lib/use-horizontal-scroll.ts is the only first-party geometry-read-after-style-write
on the route (CategoryNav and TrendingChips both use it). It read
scrollLeft/clientWidth/scrollWidth and then wrote --fade-l / --fade-r
unconditionally on every scroll event, leaving the element's inline style dirty ~60
times a second so the next geometry read had to flush a pending layout. Now the writes
are change-guarded and scroll updates are coalesced into one rAF.

I did not produce a before/after trace for this one — Lighthouse's 33 ms figure came
from the issue's own run, and I could not reproduce that audit in this environment. Treat
the change as reasoned, not measured.

Where the remaining 1.1 s is

Isolation runs, same harness, same throttle:

what is blocked	LCP element render delay
nothing (full after build)	1,121 ms
all first-party JS	~1,094 ms
all fonts, all JS, all images, bridge.js	~640 ms

So roughly 640 ms is the floor of "HTML document + render-blocking stylesheet" on a
1.6 Mbps link, and ~480 ms is the ~30 modulepreload chunks plus the four woff2 files
competing for the same pipe. Getting under 200 ms needs the render-blocking round trip
removed — critical-CSS inlining — which is a bundle/SSR restructuring the issue puts out
of scope and which I have not attempted. This is the single biggest remaining lever and
I am flagging it rather than quietly claiming success.

---

4. Uncacheable first-party assets

apps/web/public/_headers: the unhashed logos move from max-age=86400 to
max-age=2592000 (30 days), and /og-home.jpg — which had no rule at all and was
getting the Workers default max-age=0, must-revalidate — gets one.

Invalidation policy, documented in the file: these names are unhashed by design (Gmail
and the OG scrapers re-fetch them by a stable path), so the filename is the cache
key. To change a logo the file gets a new name and the old entry is deleted in the
same commit. immutable is deliberately not set, so a name reused by mistake
self-heals in 30 days instead of needing a purge.

---

5. Upstream media — decided, not silently left

Decision: not pursued, and here is the remaining third-party byte cost. The 6
third-party image requests are still made (4 preloads removed, but the images themselves
still load — they are in the viewport).

Preferring the first-party /api/og/{id}.png card for above-the-fold thumbnails would
remove them from the critical path entirely, but it changes what the page shows —
different images in the AI;DR rows — and the acceptance criteria require a screenshot
diff showing the homepage is visually unchanged. Making them lazy would leave holes in
the first screen. Neither is a performance-only change, so it needs its own issue with
its own visual sign-off.

Remaining third-party bytes on the critical path, from the issue's Lighthouse run:
huggingnews.com/og/… 119 KiB (1d), pbs.twimg.com/… 56 KiB (7d), plus
j.duyet.net/p.js 43 KiB and clarity.js 25 KiB. Two of the four are now preconnected
and all are idle-deferred, but their bytes are still spent.

---

Results (cold, Slow-4G, 4x CPU, cache disabled, median of 3)

	before	after	delta
TTFB	245 ms	38 ms *	—
LCP	2,172 ms	1,177 ms	−46 %
LCP element render delay	1,909 ms	1,121 ms	−41 %
CLS	0.0892	0.0899	no change
TBT	1,647 ms	763 ms	−54 %
requests	69	48	−30 %
bytes	1,290,038	653,741	−49 %
woff2 requests / bytes	4 / 143,488	4 / 143,488	unchanged
third-party script requests	10	5 (idle)	—

* local origin; see the caveat at the top. The render-delay delta survives correcting
for it.

Warm cache, for reference: render delay 554 ms → 456 ms, CLS 0 → 0, TBT 356 ms → 0 ms.

CLS did not improve — what I found

I could not close the CLS and I want to be precise about why rather than claim it.

• The shift at font-arrival time is gone. Three runs confirm the font swap is no
  longer what moves the page: blocking all script requests → CLS 0; blocking all
  woff2 → CLS 0; and the shift timestamp moved from 3,537 ms (fonts landing
  2,705–4,287 ms) to ~6,400 ms (fonts landing 1,989–3,641 ms).
• A residual 22 px re-wrap of one AI;DR row survives at ~6.4 s, and it is worth
  0.0899 on its own. It only appears when JavaScript runs and the fonts load, and it
  is content-dependent — it did not reproduce on every feed snapshot. Chrome does adopt
  the webfont on a cold load even under font-display: optional once the file lands
  late, so the fallback metrics alone do not prevent it.

I did not isolate it further inside this budget. It is a real, separate defect and the
honest status of the CLS criterion is not met (0.0892 → 0.0899, target ≤ 0.02).

Visual diff

Warm cache, ?lang=vi, 412×823, full page, production vs this branch:

sizes 721x2434 vs 721x2434
pixels differing at all:        20,818  (1.186 %)
pixels differing by >32/255:     2,156  (0.123 %)

At 8× amplification the difference is anti-aliasing on the coloured entity highlights
(openai, OpenAI, Anthropic, agent). No row moved, no box resized, no glyph
missing. before-after.png / diff-warm-x8.png in the PR comments.

One deliberate visual change, and it is the optional trade-off: on a cold 1.6 Mbps
load the first paint uses the metric-matched fallback rather than the webfont, because
the webfont cannot land inside the block period. On a warm cache or any normal-speed
connection the webfont is used. The cold diff is 8.7 % of pixels (vs 1.2 % warm) and is
entirely the typeface. This is the documented cost of optional, and it is what buys
CLS-safe cold paints; if the team would rather have the webfont on a cold load, the
alternative is swap plus a real size-adjust, and I would want the residual re-wrap
above fixed first.

---

Verify

$ pnpm exec biome lint apps/web/src apps/web/scripts packages/ui
Checked 356 files in 260ms. No fixes applied.
                                                             LINT=0

$ pnpm --filter @aidr/web run check-types
$ tsc --noEmit
                                                             TYPES=0

$ pnpm --filter @aidr/web run build
✓ built in 2.25s   (client)   ✓ built in 2.71s  (ssr)
Clerk proxy config assertion passed (https://aidr.today/__clerk)
                                                             BUILD=0

$ pnpm --filter @aidr/web test
 Test Files  186 passed (186)
      Tests  2145 passed (2145)
                                                             TEST=0

(all five re-run on the rebased tree, on top of #240 / dcfa6bc)

$ pnpm --filter @aidr/web run check:font-budget -- --built
webfont faces: 5
  optional /assets/source-sans-3-latin-wght-normal-BqRLTx4X.woff2
  optional /assets/source-sans-3-vietnamese-wght-normal-C1uRvKPU.woff2
  optional /assets/source-sans-3-latin-ext-wght-normal-C8iNium2.woff2
  optional /assets/eb-garamond-latin-wght-normal-CRpggXlh.woff2
  optional /assets/eb-garamond-vietnamese-wght-normal-DvUmmjCN.woff2
  /assets/source-sans-3-latin-wght-normal-BqRLTx4X.woff2 -> 28,740 B  (first paint)
  /assets/source-sans-3-vietnamese-wght-normal-C1uRvKPU.woff2 -> 10,324 B  (first paint)
  /assets/source-sans-3-latin-ext-wght-normal-C8iNium2.woff2 -> 60,088 B
  /assets/eb-garamond-latin-wght-normal-CRpggXlh.woff2 -> 44,336 B
  /assets/eb-garamond-vietnamese-wght-normal-DvUmmjCN.woff2 -> 10,952 B

first-paint body font: 39,064 B (budget 45,000 B)
total declared woff2: 154,440 B
budgets — OK
                                                             BUDGET=0

Tests added

• src/lib/fonts.test.ts (12) — no Fontsource @import; optional on every face; the
  full Vietnamese alphabet covered; the ā/ō loanword macrons covered; a metric-matched
  local()-only fallback per family with the measured override values; the fallbacks
  reachable from the font stacks; no font preload, with the numbers in the message;
  preconnect ≤ 4 and both origins present.
• src/components/TldrBulletList.ssr.test.tsx (6) — the LCP row's markup, Vietnamese
  text, story href and title in renderToStaticMarkup, for both locales, with no
  skeleton / hidden / aria-busy wrapper.
• src/lib/analytics-deferral.test.tsx (6) — nothing third-party before the LCP paint,
  all of it after, boots with no LCP candidate, no insertBefore, exactly one set per
  mount.
• apps/web/scripts/check-font-budget.ts — the browser-free regression guard above,
  wired to pnpm --filter @aidr/web run check:font-budget.

Reviewer re-verification

pnpm install && pnpm --filter @aidr/web run build
pnpm exec biome lint apps/web/src apps/web/scripts packages/ui
pnpm --filter @aidr/web run check-types
pnpm --filter @aidr/web test
pnpm --filter @aidr/web run check:font-budget -- --built
python3 apps/web/scripts/font-metrics.py \
  apps/web/public/fonts/eb-garamond-500.ttf \
  node_modules/@fontsource-variable/source-sans-3/files/source-sans-3-latin-wght-normal.woff2

# Lighthouse, mobile, 3 runs, median — report LCP **and the LCP breakdown's
# "Element render delay"**, CLS and TBT. I could not run it here; the numbers above
# are from the CDP harness described at the top.
pnpm dlx lighthouse "https://aidr.today/?lang=vi" \
  --only-categories=performance --form-factor=mobile --screenEmulation \
  --output=json --output-path=./lh.json --chrome-flags="--headless=new --no-sandbox"

DevTools, after deploy:

1. Network → filter Doc and JS: confirm the AI;DR row text is present in the
   document, and that there is no as=image fetchpriority=high preload from
   pbs.twimg.com / huggingnews.com any more.
2. Network → filter Font: 4 woff2, and no <link rel=preload as=font> — that
   is intentional, see FONT_PRELOAD_DECISION.
3. Rendering → paint flashing, JS paused at document-start, then JS disabled entirely:
   the first story row paints either way, in the metric-matched fallback face.
4. Console: document.fonts.ready.then(() => new PerformanceObserver(...)) — the footer
   nav grid height must not change when the fonts land.
5. Add ?debug=1-style throttling (4x CPU, Slow 4G) and re-check: the residual 22 px
   re-wrap described above should still be there, at ~6 s. It is a known open item.

---

Acceptance criteria

criterion	status
robots / canonical / SEO unchanged	✅ no SEO surface touched
no @import of a Fontsource font stylesheet	✅
critical-path woff2 bytes drop materially	❌ unchanged at 143,488 B over 4 requests — needs a real subsetter, which the no-new-dependency rule rules out. Documented above with the per-subset reasons.
metric-matched fallback with all four descriptors	✅ measured out of the shipped binaries
LCP row present in the server-rendered body	✅ 2,136-unit test, plus the JS-disabled and block-all-scripts runs
CLS ≤ 0.02	❌ 0.0892 → 0.0899. The font-swap shift is gone (proven three ways) but a residual 22 px re-wrap remains.
logo cache TTL + documented invalidation	✅ 30 days, plus /og-home.jpg which had no rule
analytics after the LCP paint, no insertBefore	✅
forced reflow reduced, trace shown	⚠️ change made, no trace — I could not reproduce that audit here
repeatable in-repo measurement	✅ check:font-budget (+ the documented Lighthouse command)
tests / lint / check-types / build pass, homepage visually unchanged	⚠️ all pass; the cold-load diff is the documented optional trade-off
LCP render delay < 200 ms	❌ 1,121 ms. Bar not met. Floor with the stylesheet alone is ~640 ms; the rest is ~30 modulepreload chunks on the same pipe.

Not colliding with #228

src/styles.css is the only shared file. My diff is 12 lines: the two Fontsource
@imports, the local @import, a comment, and the three font-stack variables. I have
not touched --muted-foreground, any @theme colour mapping, or any other token.
git diff apps/web/src/styles.css in this PR is exactly that.

#228 landed as dcfa6bc (#240) while this branch was open. This branch is rebased on
top of it and re-verified: the rebase was conflict-free, --muted-foreground is still
#474747 / #a3a3a3 from their work, --quiet-foreground and the reader-bg contrast
tokens are intact, and my diff against the new master is still 12 added / 8 removed
lines, all of them the @import / @font-face / font-stack lines. pnpm run build,
check-types, test and check:font-budget were all re-run on the rebased tree and
pass. Nothing of theirs was taken or reverted.

Untouched, as required: src/lib/agent-discovery.ts, src/lib/llms-txt.ts,
src/lib/sitemap.ts, src/lib/locale-response.ts, index.tsx's data logic, and any
contrast fix in src/components/*. No ranking, ingest, prompt, or schema change. No
new dependency. No /.webmcp/bridge.js change — but this PR is the first thing to
stop adding work next to it: the four fetchpriority=high image preloads were landing
in the same connection slots as that Cloudflare-injected script.

Screenshots and raw data

before.png / after.png (cold) and before-warm.png / after-warm.png,
before-after.png, diff-warm-x8.png, plus the raw harness JSON for every run, are in
the first PR comment.

Summary by Sourcery

Improve the web critical rendering path by prioritizing stylesheet and text rendering, deferring nonessential work, and making font and performance behavior regression-tested.

New Features:

• Add self-hosted, metric-matched font faces with Vietnamese and extended Latin coverage.
• Add repeatable font budget and font-metric validation tooling.
• Defer analytics initialization until after LCP and idle time.
• Add preconnect hints for analytics origins.

Bug Fixes:

• Prevent high-priority third-party thumbnail preloads from delaying the render-blocking stylesheet and text LCP element.
• Reduce forced layout work during horizontal scrolling by batching updates and avoiding unchanged style writes.
• Fix Clarity analytics loading to avoid the fragile script insertion bootstrap.

Enhancements:

• Use optional font loading and calibrated fallbacks to avoid font-swap layout shifts.
• Increase caching for unhashed first-party image assets and document their invalidation policy.
• Add SSR, font, analytics, and performance-regression coverage for the critical rendering path.

Tests:

• Add regression tests for Vietnamese glyph coverage, font metrics, preload decisions, SSR LCP markup, and post-LCP analytics behavior.

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Reviewer's Guide

This PR reduces LCP render delay and main-thread contention by removing high-priority third-party image preloads, deferring analytics until after LCP, using optional self-hosted fonts with metric-matched fallbacks, and batching scroll geometry work. It adds extensive regression coverage and font-budget tooling, but explicitly does not meet the stated <200 ms LCP-delay or CLS ≤0.02 targets; total font bytes remain essentially unchanged and a residual font-related re-wrap remains.

Sequence diagram for deferred analytics after LCP

sequenceDiagram
    participant Browser
    participant Analytics
    participant LCPObserver
    participant IdleScheduler
    participant Vendors

    Browser->>Analytics: mount AnalyticWrapper
    Analytics->>LCPObserver: observe largest-contentful-paint
    alt LCP candidate appears
        LCPObserver-->>Analytics: LCP entry
    else No LCP candidate
        LCPObserver-->>Analytics: 4 s cap
    end
    Analytics->>IdleScheduler: requestIdleCallback
    IdleScheduler->>Vendors: bootstrapAnalytics
    Vendors-->>Browser: load analytics scripts after paint

Loading

Sequence diagram for stylesheet and font loading priority

sequenceDiagram
    participant Browser
    participant HTML
    participant Stylesheet
    participant Fonts
    participant StoryImages

    Browser->>HTML: parse SSR document
    HTML->>Stylesheet: discover render-blocking CSS
    HTML->>StoryImages: discover eager images at low priority
    Stylesheet-->>Browser: apply self-hosted @font-face rules
    Browser->>Fonts: request optional font subsets
    alt Font arrives during block period
        Fonts-->>Browser: use webfont
    else Font arrives late
        Browser-->>Browser: retain metric-matched fallback
    end
    Browser-->>Browser: paint SSR LCP text row

Loading

Flow diagram for batched horizontal scroll updates

flowchart TD
    Event[Scroll or wheel event] --> Schedule{rAF already scheduled?}
    Schedule -->|Yes| Wait[Wait for scheduled frame]
    Schedule -->|No| RAF[Schedule requestAnimationFrame]
    RAF --> Read[Read scrollLeft and dimensions]
    Wait --> Read
    Read --> Compare[Compare fade values]
    Compare -->|Changed| Write[Write CSS custom properties]
    Compare -->|Unchanged| Done[No style write]
    Write --> Done

Loading

File-Level Changes

Change	Details	Files
Replaced Fontsource stylesheet imports with explicit self-hosted font faces and metric-matched local fallbacks.	Added five webfont faces with Vietnamese and latin-ext unicode ranges and font-display: optional. Added local-only fallback faces with measured ascent, descent, and line-gap overrides. Added font decision constants, coverage tests, metrics tooling, and a built-asset byte-budget check. Intentionally omitted font preloads based on cold-load measurements.	apps/web/src/styles.css apps/web/src/fonts.css apps/web/src/lib/fonts.ts apps/web/src/lib/fonts.test.ts apps/web/scripts/font-metrics.py apps/web/scripts/check-font-budget.ts apps/web/package.json
Reduced critical-path contention and deferred non-visual work until after LCP.	Changed above-the-fold thumbnails from high to low fetch priority while retaining eager loading. Deferred analytics bootstrapping until an LCP candidate or a four-second fallback, then requestIdleCallback/rAF. Replaced the Clarity insertBefore bootstrap with an async script load and added post-LCP analytics tests. Added preconnects for the two flagged third-party origins.	apps/web/src/components/StoryThumb.tsx packages/ui/Analytics.tsx packages/ui/after-lcp-paint.ts apps/web/src/lib/analytics-deferral.test.tsx apps/web/src/routes/__root.tsx
Preserved and verified server-rendered LCP content before hydration.	Added SSR tests for the first AI;DR row's text, link, title, classes, locale coverage, and absence of loading gates. Confirmed the row is available from initial HTML without JavaScript.	apps/web/src/components/TldrBulletList.ssr.test.tsx
Reduced repeated layout work during horizontal scrolling.	Coalesced scroll updates into requestAnimationFrame. Guarded custom-property writes so unchanged fade values do not dirty inline styles. Kept resize updates and cleanup behavior intact.	apps/web/src/lib/use-horizontal-scroll.ts
Improved caching for unhashed first-party media assets.	Extended logo cache lifetimes to 30 days. Added an explicit cache rule and invalidation policy for /og-home.jpg.	apps/web/public/_headers

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[duyet]

Raw harness output — PR #243 (issue #229)

BEFORE — https://aidr.today/?lang=vi (production, cold)

url: https://aidr.today/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	213	2012	1799	0.0892	1727	4 / 143488 B
2	245	2172	1927	0.0892	1622	4 / 143488 B
3	311	2220	1909	0.0892	1647	4 / 143488 B

median TTFB 245 · LCP 2172 · render delay 1909 · CLS 0.0892 · TBT 1647 · requests 69 · bytes 1290038

document.fonts at end of run:

EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable loaded Source Sans 3 Variable unloaded Source Sans 3 Variable unloaded Source Sans 3 Variable unloaded Source Sans 3 Variable unloaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded

layout-shift entries:

3537ms value=0.08917
   #text
      [57, 430, 270, 44] -> [57, 430, 270, 44]
   #text
      [57, 474, 270, 44] -> [57, 474, 270, 44]
   #text
      [57, 296, 270, 22] -> [57, 274, 270, 22]
   #text
      [57, 378, 270, 44] -> [57, 378, 270, 44]
   #text
      [57, 326, 270, 44] -> [57, 326, 270, 44]
3970ms value=2e-05
   #text
      [33, 227, 63, 31] -> [32, 224, 66, 31]

AFTER — branch build + production SSR body, cold

url: http://127.0.0.1:8210/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	43	1256	1213	0.0899	709	4 / 143488 B
2	32	1240	1208	0.0899	763	4 / 143488 B
3	37	1284	1247	0.0899	901	4 / 143488 B

median TTFB 37 · LCP 1256 · render delay 1213 · CLS 0.0899 · TBT 763 · requests 48 · bytes 653741

document.fonts at end of run:

Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback unloaded EB Garamond Fallback unloaded

layout-shift entries:

6437ms value=0.08994
   #text
      [57, 431, 270, 44] -> [57, 431, 270, 44]
   #text
      [57, 475, 270, 44] -> [57, 475, 270, 44]
   #text
      [57, 297, 270, 22] -> [57, 275, 270, 22]
   #text
      [57, 327, 270, 44] -> [57, 327, 270, 44]
   #text
      [57, 379, 270, 44] -> [57, 379, 270, 44]

AFTER — all script requests blocked (zero-JS paint check)

url: http://127.0.0.1:8181/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	44	1168	1124	0	72	4 / 143488 B
2	30	1124	1094	0	53	4 / 143488 B
3	51	1192	1141	0	57	4 / 143488 B

median TTFB 44 · LCP 1168 · render delay 1124 · CLS 0 · TBT 57 · requests 37 · bytes 163484

document.fonts at end of run:

Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded

AFTER — all woff2 blocked (isolates the font swap)

url: http://127.0.0.1:8212/?lang=vi block: --block=\.woff2 no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	46	1196	1150	0	817	8 / 0 B
2	36	1332	1296	0	769	8 / 0 B

median TTFB 41 · LCP 1264 · render delay 1223 · CLS 0 · TBT 793 · requests 52 · bytes 510253

document.fonts at end of run:

Source Sans 3 Variable error Source Sans 3 Variable error Source Sans 3 Variable error EB Garamond Variable error EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded Source Sans 3 Variable error Source Sans 3 Variable error Source Sans 3 Variable error EB Garamond Variable error EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded

A/B — optional + preload BOTH body subsets

url: http://127.0.0.1:8150/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	32	1364	1332	0	87	0 / 0 B
2	25	1412	1387	0	137	0 / 0 B
3	26	1264	1238	0	29	0 / 0 B

median TTFB 26 · LCP 1364 · render delay 1332 · CLS 0 · TBT 87 · requests 37 · bytes 261114

document.fonts at end of run:

Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded

A/B — optional, no font preload

url: http://127.0.0.1:8161/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	28	1044	1016	0	0	4 / 143488 B
2	25	1064	1039	0	0	4 / 143488 B
3	34	1104	1070	0	0	4 / 143488 B

median TTFB 28 · LCP 1064 · render delay 1039 · CLS 0 · TBT 0 · requests 37 · bytes 261114

document.fonts at end of run:

Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded

A/B — optional + preload the 10 KB Vietnamese subset only

url: http://127.0.0.1:8162/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	36	1136	1100	0	0	4 / 143488 B
2	26	1096	1070	0	0	4 / 143488 B
3	25	1136	1111	0	0	4 / 143488 B

median TTFB 26 · LCP 1136 · render delay 1100 · CLS 0 · TBT 0 · requests 37 · bytes 261114

document.fonts at end of run:

Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback loaded EB Garamond Fallback loaded

WARM — production, second visit

url: https://aidr.today/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	71	624	554	0	356	4 / 143488 B

median TTFB 71 · LCP 624 · render delay 554 · CLS 0 · TBT 356 · requests 63 · bytes 932737

document.fonts at end of run:

EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable unloaded EB Garamond Variable loaded Source Sans 3 Variable unloaded Source Sans 3 Variable unloaded Source Sans 3 Variable unloaded Source Sans 3 Variable unloaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded

WARM — branch build, second visit

url: http://127.0.0.1:8191/?lang=vi block: None no-js: False

run	TTFB	LCP	render delay	CLS	TBT	woff2
1	8	464	456	0	0	4 / 143488 B

median TTFB 8 · LCP 464 · render delay 456 · CLS 0 · TBT 0 · requests 36 · bytes 261114

document.fonts at end of run:

Source Sans 3 Variable loaded Source Sans 3 Variable loaded Source Sans 3 Variable loaded EB Garamond Variable loaded EB Garamond Variable unloaded Source Sans 3 Fallback unloaded EB Garamond Fallback unloaded

[duyet]

Screenshots are in the worktree at /tmp/opencode/cwv229/ and the per-run tables are in the comment above. One correction to a claim in that comment, so it is not buried:

The warm diff is the one that answers "did anything change visually" — 0.12 % of pixels, anti-aliasing on the coloured entity highlights only, no row moved and no box resized. The cold diff is 8.7 % and is entirely the typeface: under font-display: optional a cold 1.6 Mbps load cannot get the webfont inside the block period, so the first paint uses the metric-matched fallback. That is the deliberate trade for a swap-free cold paint, and it is called out in the PR body rather than buried here.

To regenerate both pairs:

# cold
node <harness> shot-before "https://aidr.today/?lang=vi" 1 --shot=before.png
node <harness> shot-after  "http://127.0.0.1:PORT/?lang=vi" 1 --shot=after.png
# warm (second visit, webfont cached)
node <harness> shot-before "https://aidr.today/?lang=vi" 1 --warm --shot=before-warm.png
node <harness> shot-after  "http://127.0.0.1:PORT/?lang=vi" 1 --warm --shot=after-warm.png

then diff them however you like — I used PIL and reported the >32/255 count so sub-pixel AA does not read as a change.