Segment filter editor in the UI is really slow with several million contacts #487

Description

To Reproduce

• Create 2M to 5M contacts (or more), with different custom fields (e.g country, platform, last_visited, ...) say 30 fields
• Go to Segments, click on "Create Segment"
• Look at "Filter conditions"

Current behaviour

"Loading available fields and events..." is displayed for almost a minute

Expected behavior

Filter conditions pop up and are editable near instantly

Environment

Please select the option that applies:

• Deployment type:

  • Hosted
  • Self-hosted

If self-hosted, please provide relevant details (Docker, Kubernetes, bare metal, etc.):

kubernetes with custom beefy database (so db being small is not the bottleneck, the scan is)

Version verification

Using next version I could reproduce.

• I have confirmed this issue still exists on the latest commit (not the latest tag)

  • Commit SHA tested: 1df7bfb3a5c6bb5bf99301daa5dda1e8a80d2655

Checklist

Please confirm the following before submitting:

• I have searched existing issues to ensure this bug has not already been reported
• I have provided clear reproduction steps
• I have included all relevant environment details