The performance patterns post covers what to optimize. This one covers verifying those wins survived the real world: collecting INP, LCP, and CLS from actual users, interpreting p75 instead of median, and blocking regressions in CI before they reach production.

I added React.memo around a heavy list component, ran the profiler, and watched the render time drop from 42ms to 8ms. Shipped it. Two weeks later a stakeholder asked why the page still felt sluggish on mobile. The profiler had been running on my M2 MacBook with no network throttling and a warm cache. Real users were on mid-range Android phones on 4G in areas with variable signal.

The optimization was real. The measurement was wrong. This post is about closing that gap: collecting Web Vitals from actual users, reading the numbers that matter (p75, not median), and catching regressions in CI before they ship.

Why the React Profiler and Lighthouse in DevTools aren't enough

The React Profiler measures JavaScript execution time on your machine.…