First Input Delay (FID) measured the time between a user’s first interaction with a page — clicking a button, tapping a link — and when the browser could begin processing that event. It was a Core Web Vitals metric used by Google as part of the Page Experience signals. A good FID score was under 100 milliseconds. FID was retired as an official Core Web Vital in March 2024 and replaced by Interaction to Next Paint (INP), which measures responsiveness across the entire page visit rather than only the first interaction. Legacy reports in Google Search Console and tooling may still reference FID, but all new optimisation efforts should target INP instead.