INP (Interaction to Next Paint)
INP measures real responsiveness: the delay between an interaction — click, keystroke — and the next visible update. Successor to FID since March 2024, it must stay below 200 ms.
In practice
A degraded INP reveals a blocked main thread: third-party scripts, long tasks, heavy hydration. Diagnosis comes from field data — real user measurements — more than from lab tests, as INP only surfaces under actual use. Breaking up tasks, deferring the non-essential, lightening event listeners: the recipe is known, rarely applied.