Interaction to Next Paint became a Core Web Vital in March 2024, replacing First Input Delay. The replacement happened because FID only measured how long the browser waited before starting to handle the first interaction. A page could score well on FID and still freeze for a second every time someone opened a menu.
INP measures the whole path. It starts when the user clicks, taps, or presses a key, includes the input delay, the event handler's own work, and the rendering that follows, and ends when the browser paints the next frame. It then reports roughly the worst interaction across the visit rather than the first one.
The thresholds are 200 milliseconds for good and 500 milliseconds for poor, again at the 75th percentile.
The usual cause is long JavaScript tasks blocking the main thread. Breaking heavy handlers into smaller pieces, yielding to the browser between them, moving computation into a worker, and cutting third-party scripts that run on interaction all move the number. Reducing DOM size helps too, because rendering cost sits inside the measurement.