Interaction to Next Paint (INP) measures the latency between a user interaction (click, tap, keypress) and the next paint — the first visible response on screen. INP replaced First Input Delay (FID) as a Core Web Vital in March 2024.
Context
INP captures responsiveness throughout a session, not just the first interaction. A site that feels 'laggy' halfway through a form fill or checkout is INP-failing even if the initial page load felt fast.
The threshold for good INP is under 200ms for 75% of sessions. The most common cause of poor INP is heavy JavaScript execution blocking the main thread during interaction.
A common INP failure: clicking an 'Add to Cart' button that fires a 400ms analytics call synchronously before the UI updates. The 400ms wait blocks the paint, INP fails, and the user perceives the site as slow even though actual network requests are fast.
INP is measured on interactions that occur — if your page has no interactive elements on a particular session, no INP score is recorded. This means INP improvements don't always show up in lab tests; you need real-user monitoring.
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