First Input Delay (FID) measured the time between a user's first interaction — a click, tap, or key press — and the moment the browser actually started processing the event handler. It was a Core Web Vital from 2020 until March 2024, when Google replaced it with INP as the official responsiveness metric.
Why it matters
FID is now a legacy metric: as of 12 March 2024, FID was removed from Core Web Vitals and is no longer used for page experience signals. Treat references to it in older audits and dashboards as historical context. INP is a stricter test because it samples the slowest interaction across the whole page lifecycle, not just the first one — a site with a green FID can still post a poor INP.
How to migrate
- Replace FID dashboards and alerts with INP targets (good is under 200 ms at the 75th percentile).
- Keep watching TBT in the lab — it is the closest synthetic proxy for INP responsiveness.
- Audit long tasks on the main thread and break them up with
scheduler.yield()or Web Workers. - Defer non-critical JavaScript and load third-party widgets after interaction.
- Hydrate framework components lazily so the main thread stays free during the first seconds.
- Re-instrument RUM using the
web-vitalslibrary, which now reports INP by default.