First Input Delay (FID) measured the time from when a user first interacted with a page (clicked a button, tapped a link) to when the browser was actually able to begin processing the event handler. It was a Core Web Vital from 2020 to 2024.
Google's old FID thresholds: under 100ms was "Good". Most pages passed FID easily because the first interaction usually happened after the main thread had quieted, but real-user responsiveness was often poor across the rest of the session.
FID was replaced by Interaction to Next Paint (INP) on 2024-03-12. INP measures every interaction, not just the first, and includes handler-execution + presentation delay -- a much fuller picture of perceived responsiveness. Migrate any FID-based monitoring to INP.