Interaction to Next PaintINP

Interaction to Next Paint measures how quickly a page visually responds to user interactions, taps, clicks, and key presses, across the whole session. It reports a value near the worst interaction, so it captures sustained responsiveness rather than the one-time latency of the first input. The good threshold is 200 milliseconds or less at the 75th percentile.

INP became an official Core Web Vital on March 12, 2024, replacing First Input Delay. FID only measured the delay before the browser began processing the first interaction, which let janky-but-fast-to-start pages pass. INP is harder to satisfy because it accounts for processing time and rendering for every interaction.

The usual causes of poor INP are long JavaScript tasks that block the main thread, heavy event handlers, and large rendering work triggered by interaction. The fixes are to break up long tasks, defer non-critical JavaScript, yield to the main thread, and minimize the DOM work an interaction triggers. For React apps this often means avoiding expensive synchronous state updates on every keystroke and using techniques like deferred updates and memoization where they genuinely help.