Interaction to Next Paint (INP)
Core Web Vital measuring how long the page takes to visually respond to a user interaction. Target: under 200ms.
In long form.
INP replaced First Input Delay (FID) as a Core Web Vital in March 2024. Where FID measured only the first interaction's input delay, INP measures the latency of all interactions throughout the page lifetime, reporting roughly the worst (specifically the 98th percentile). It captures the full pipeline: input delay, processing time, and presentation delay. Common causes of poor INP: heavy JavaScript on the main thread, large component re-renders, unsharded event handlers.
INP is harder to fix than LCP. Wins usually come from breaking up long tasks, deferring third-party scripts, and using `requestIdleCallback` or `useDeferredValue` to schedule non-critical updates off the main interaction path.
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