Interaction to Next Paint (INP)
How long the page takes to show a response after a tap or a click, across the whole visit. Good is 200 milliseconds or less.
It replaced First Input Delay because the first interaction was never the problem. INP looks at the slowest interactions of the visit, which is where a page that felt fine at the top turns sticky halfway down.
The cause is almost always JavaScript holding the main thread: a large bundle parsing, a handler doing work on every keystroke, a third-party script waking up. Less script on the page is the fix that keeps working.