Definition: A browser metric that measured the delay between a user's first interaction with a page and the moment the browser could begin handling it. It was replaced by Interaction to Next Paint (INP) as a Core Web Vital in 2024.
So what? Because it covered only the first input, it could miss slow responses later in a session, which is why INP replaced it.
Example: A page that responds quickly to the first click but becomes sluggish once more scripts load would score well on FID yet still feel slow to use.
