PerfLens

Live Web Vitals per screen, the element causing each issue, and the fix. Runs only where you ask it to. Nothing leaves your browser.

PerfLens shows you why a page feels slow, while you are looking at it. It measures the Core Web Vitals (LCP, CLS and INP, plus FCP and TTFB) for the screen you are on. For each problem it points to the element or component causing it and suggests a fix. WHAT YOU GET • Live Web Vitals per screen, with a clear verdict: fast, getting slow, or slow for users • The exact element behind each issue (the LCP element, layout-shift causers, slow interactions) • A plain-language reason and a suggested fix for every issue • Page outlines that highlight the offending elements on the page • Re-render counts for React apps, and optional function-level freeze profiling • Quiet mode: the pill appears only when a screen has an issue • Optional console logging: one collapsed group per new issue • Shortcut Alt+Shift+W to open or close the panel YOU STAY IN CONTROL • Runs automatically on localhost and 127.0.0.1 only • On any other site, it runs only when you click "Run on this page" or turn on "Always on" for that host. Permission is requested once per host. • Nothing leaves your browser. There is no telemetry and no analytics. • The optional fix-server setting accepts loopback addresses only. • The extension never edits your files. HOW TO USE 1. Open the page you want to check. 2. Click the PerfLens icon and choose "Run on this page". 3. Open the panel to see each issue, its cause, and the fix. For developers who want to find and fix web performance problems before users notice them.