First Input Delay (FID)
First Input Delay (FID) measures interactivity on a web page. The time it takes before the user can provide input.

First Input Delay is part of the core web vitals as one of the three main metrics used to measure site speed. Learn more on how you can design for site speed.
Have a look at the factors that Impact LCP, FID/TBT, and CLS scores and research on the state of Core Web Vitals here.