FID measured only the delay before the first interaction began processing; INP measures the whole response — input, processing, and the time to paint the result — across every interaction of the visit, reporting the worst-ish value. A page that feels snappy on the first click but stutters on the tenth is now measured honestly. Good is under 200ms, needs work up to 500ms, poor beyond.
The usual offenders are long tasks on the main thread: heavy JavaScript hydration, big re-renders, oversized event handlers. The fixes are the standard main-thread diet — less JavaScript, split bundles, deferred non-critical work — plus optimizing the handlers themselves: yield to paint, do work in chunks, keep the DOM small where the user actually interacts.
Related terms
Core Web Vitals
Google's three field metrics for loading, responsiveness and visual stability, measured on real visits rather than in a lab.
LCP (Largest Contentful Paint)
The moment the biggest piece of content in the viewport finishes rendering — in practice, when the page looks loaded.
CLS
Cumulative Layout Shift — the Core Web Vital measuring how much the page jumps around while loading.
Hydration
The step where JavaScript takes over server-rendered HTML in the browser and makes it interactive.
The bench this belongs to
Web performanceA rebuild that loads on a phone on mobile data, in the country your customers are actually in. Core Web Vitals are the measurement, revenue is the point.
