INP measures the time from when a user interacts with your page—clicking a button, tapping a link, typing in a form—to when the browser paints the next visual update. It captures the full lifecycle: input delay, processing time, and rendering delay. Google considers anything under 200ms good, 200-500ms needs improvement, and over 500ms poor. Unlike First Input Delay (FID), which only measured the delay before processing started and only tracked the first interaction, INP observes all interactions throughout the page's lifecycle and reports the worst (or near-worst) one. This makes it far more representative of real user frustration. A page might have a perfect FID but still feel sluggish if subsequent clicks take 800ms to show feedback. Common INP killers include heavy JavaScript execution blocking the main thread, large DOM trees (over 1,500 nodes), third-party scripts that monopolize processing time, and inefficient event handlers. We see this constantly with WordPress sites running page builders and multiple tracking pixels—the browser is too busy parsing scripts to respond to clicks. To fix INP issues, break up long tasks into smaller chunks using requestIdleCallback or setTimeout, defer non-critical JavaScript, minimize DOM size, and use web workers to move heavy computation off the main thread. For ecommerce sites with complex filtering or checkout flows, debouncing input handlers and optimizing React or Vue component re-renders typically yields the biggest improvements. At Ottawa SEO, we run Chrome DevTools Performance profiling on key interaction points—mobile menu toggles, filter buttons, form submissions—to identify which scripts block the main thread longest. We've seen INP drop from 650ms to 180ms just by deferring Google Tag Manager and lazy-loading chat widgets until after the first user scroll. Real User Monitoring through Chrome UX Report or tools like SpeedCurve shows you the 75th percentile INP Google actually uses for ranking, not just lab data.