Interaction to Next Paint (INP)
A Core Web Vital metric that measures the responsiveness of a page by tracking the latency of all user interactions (clicks, taps, key presses) throughout the page lifecycle and reporting a value representative of the worst interactions.
INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. While FID only measured the delay of the first interaction, INP captures responsiveness across the entire page visit. An INP of 200 milliseconds or less is considered 'good.'
INP measures the full round trip: from the user interaction, through event processing (JavaScript execution), to the browser rendering the next visual update. Poor INP is often caused by long-running JavaScript tasks, excessive DOM manipulation, or main-thread-blocking operations during event handlers.
Why It Matters for QA Teams
Users expect instant feedback when they click or type. A sluggish response to interactions makes a website feel broken even if the content loads quickly. INP measures exactly this perceived sluggishness.
Example
Clicking the 'Filter' button on a product listing page triggers a JavaScript function that re-sorts 3,000 DOM elements synchronously. The INP for this interaction is 680ms, during which the page appears frozen. The fix: implement virtual scrolling to render only visible items and process the sort in smaller chunks using requestIdleCallback, reducing INP to 90ms.