Interaction to Next Paint (INP)
A Core Web Vital that measures how quickly the page visually responds to the user's interactions (clicks, taps, key presses) — from input to the next painted frame. It measures responsiveness/the feeling of "lag". It replaced First Input Delay (FID) in 2024. Target: under 200 ms.
What it measures: for each interaction it measures the time from the user acting until the screen updates in response; INP reports (roughly) the worst interaction during the visit → it captures "the app feels sluggish/laggy". Difference from FID (which it replaced): FID only measured the delay of the FIRST interaction and only the input delay; INP measures ALL interactions and all the way to the next paint → much more representative. Common cause: a blocked main thread — heavy JavaScript that runs for a long time prevents the browser from responding and painting. Optimization: break up long tasks (yield to the main thread, scheduler.yield), move heavy work to web workers, avoid unnecessary rendering, and defer non-critical work (requestIdleCallback). One of three Core Web Vitals and often the hardest to fix. Related to Largest Contentful Paint and requestIdleCallback.