What changed
In March 2024 Google replaced First Input Delay (FID) with Interaction to Next Paint (INP) as one of the three Core Web Vitals — and yes, it's part of the ranking signal. If your old FID was green that tells you nothing — INP can be deep in the red because it's a fundamentally different measurement. FID measured the delay before the browser started processing the first click. INP measures total time from click to the moment the browser actually paints the result, across every interaction, not just the first.
Targets
- INP ≤ 200ms — good
- INP 200–500ms — needs improvement
- INP > 500ms — poor
How to measure in production
Lighthouse in DevTools is synthetic — its INP estimate is approximate. Real numbers come only from field data, i.e. real users. Two sources matter: CrUX (the Chrome User Experience Report, surfaced in PageSpeed Insights and GSC) and your own RUM. Site Metrics Tool pulls CrUX automatically for any connected project and shows the 75th percentile over the last 28 days — the same number Google uses for ranking.
Top 5 causes of poor INP
- Long main-thread work on click: heavy handlers, synchronous setState over a large tree, unoptimised React renders.
- Third-party scripts (analytics, chats): they block the main thread — use `defer` or move them to a web worker.
- Long lists without virtualisation: render 30 visible rows via react-window or TanStack Virtual instead of 1000.
- Switching expensive tabs: wrap the state change in `startTransition` so the transition does not block the UI.
- Slow CSS animations on high-paint elements: simplify selectors, animate transform/opacity instead of top/left.
Next.js-specific notes
Next 15+ App Router defaults to Server Components, which trims the JS bundle and improves INP out of the box. If you're still on Pages Router, migrating to App Router typically yields a 30–50% INP improvement with no other code changes. Additionally: use `next/dynamic` with `ssr: false` for widgets that aren't critical to first paint, and prefetch critical routes via `prefetch`.
Frequently asked
Does INP affect SEO?
Yes. Since March 2024 INP is an official Google ranking signal as part of Page Experience. The impact isn't catastrophic — one of dozens of factors — but all else equal, a page with green INP outranks one with red.
Can I simulate INP in tests?
Lighthouse approximates it; Chrome DevTools Performance measures individual interactions exactly. But base your optimisation work on field data (CrUX) — real users run on weaker devices than your CI.