INP (Interaction to Next Paint): The Metric That Replaced FID

INP (Interaction to Next Paint) became a Core Web Vital in March 2024, replacing First Input Delay. It is a tougher, more honest measure of how responsive your page feels — and many sites that comfortably passed FID now struggle with INP. The target is 200 milliseconds or less.

What it measures (and why it is stricter than FID)

INP looks at the latency of all interactions during a visit — clicks, taps, key presses — and reports a value close to the worst one. For each interaction it measures the full time from the input until the browser paints the next frame showing a response. FID only measured the input delay of the first interaction; INP measures the entire response time of every interaction, which is far closer to what users actually experience.

Why sites fail INP

Almost always the same root cause: too much JavaScript blocking the main thread. When the browser is busy running scripts, it cannot respond to the user quickly. The biggest offenders are usually heavy first-party bundles and third-party tags — analytics, A/B testing, chat widgets, and ad scripts — that run long tasks on the main thread.

How to improve it

1. Ship less JavaScript. Remove unused scripts, code-split so each page loads only what it needs, and question every third-party tag.
2. Break up long tasks. Split work into smaller chunks and yield to the main thread (e.g. with scheduler.yield() or setTimeout) so the browser can respond between them.
3. Defer non-essential work until after the interaction, or to idle time with requestIdleCallback.
4. Optimize event handlers — debounce expensive work, and avoid heavy synchronous operations directly on input.
5. Audit third-party scripts ruthlessly. They often dominate main-thread time; load them lazily or remove them.

Common mistakes

• Assuming a good FID score means a good INP score — it often does not.
• Blaming the server when the problem is client-side script execution.
• Adding "just one more" tag manager script and wondering why responsiveness slips.

Related: Speed up WordPress · Core Web Vitals FAQ