Perflex

About

Perflex tells you exactly which line of JavaScript is making your site slow —
  and how to fix it.

  It passively instruments any page you visit and attributes main-thread time
  down to the originating script, function, and source position. It
  pattern-matches your session against 30+ known performance anti-patterns and
  generates concrete remediation plans with before/after code, a risk level, and
  a note on why the fix is safe for your UI and business logic.

  FEATURES
  • Function-level attribution via the Long Animation Frames API
  • Script leaderboard ranked by main-thread time, long tasks, transfer size &
  more
  • Live Core Web Vitals (LCP, INP, CLS), TBT, FPS, JS heap, and a 0–100 health
  score
  • Zoomable interaction timeline with causal chains (click → task → fetch → 
  reflow → shift )
  • 30+ anti-pattern detectors across loading, execution, rendering, network,
  third-party & frameworks
  • Framework-aware: detects React/Vue/Angular/Next/Nuxt and flags dev builds
  shipped to production
  • Business-safe remediations with before/after code and risk levels
  • Optional AI remediation using your own Anthropic Claude API key
  • In-page overlay (Ctrl+Shift+X) with live FPS / heap / long tasks
  • Export to JSON, HAR, OpenTelemetry traces, and PDF; shareable session links

  PRIVACY
  Everything runs locally in your browser. No account, no tracking, nothing is
  uploaded. The optional AI feature sends only an anonymized, PII-free summary
  to Anthropic — and only when you click "AI Analysis" with your own API key
  configured.

  Perflex is free and open source (MIT). Source:
  https://github.com/svemulapati/perflex