Perflex

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 37 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 • Local Lighthouse performance-score estimate with a "What If" fix simulator • Before/after Compare tab: save sessions, star a per-site baseline, and diff health, vitals, findings & per-script cost after your fix • One-click shareable Score Card — a PNG of your site's health grade and Core Web Vitals for Slack or social • Network waterfall: virtualized, color-coded timing phases, render-blocking & cache indicators • Third-party impact dashboard: vendors ranked by their performance tax, with a "remove this vendor" simulator • Zoomable interaction timeline with causal chains (click → task → fetch → reflow → shift) • 37 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 • AI Performance Coach + per-finding AI fixes — use your own Anthropic Claude OR free Google Gemini key • In-page overlay (Ctrl+Shift+X) with live FPS / heap / long tasks • Export to JSON, HAR, OpenTelemetry traces, a redesigned PDF report, and an interactive shareable HTML PRIVACY Everything runs locally in your browser. No account, no tracking, nothing is uploaded. The optional AI features send only an anonymized, PII-free summary to your chosen provider (Anthropic or Google) — and only when you use them with your own API key configured. Perflex is free and open source (MIT). Source: https://github.com/svemulapati/perflex