FID (First Input Delay)
Sitecheck Team
A legacy responsiveness metric measuring delay before the browser handles user input.
FID (First Input Delay) measured the delay between a user interaction and when the browser started processing it. It has been replaced by INP in Core Web Vitals because INP captures overall interaction responsiveness better.
Why it matters: Sites that were slow to respond felt laggy, and this metric highlighted main-thread blocking issues.
Quick tips:
- Reduce long tasks on the main thread.
- Defer non-critical JavaScript.
- Break up heavy work using scheduling APIs.
See also: INP, TBT, Core Web Vitals.