What is the difference between First Input Delay (FID) and INP?

FID (First Input Delay) was replaced by INP (Interaction to Next Paint) in March 2024. Understanding the difference helps explain why INP is better. FID: measured only the first user interaction on a page — the delay from input event to the start of event handler processing. It excluded processing time and presentation time. Problematic: a page could have fast FID (first click was fast) but terrible responsiveness throughout the rest of the session. INP: measures all user interactions (clicks, taps, key presses) throughout the full page visit. Reports the worst-case interaction latency (98th percentile). Includes input delay + processing time + presentation delay. Much more representative of real user experience. INP is harder to achieve: FID could be gamed by ensuring the first interaction happened during an idle period. INP requires the entire page to be responsive at all times. Common INP issues that FID missed: React rendering triggered by clicks blocking the main thread, animation frame callbacks delaying paint, synchronous operations in click handlers.