What it is: Measures the time from first interaction with your page to the time when their browser begins that interaction. In other words, when a user clicks a button on your page, how long does it take for the action to start happening.
Goal: FID should be 100 milliseconds or less. More than 300ms and Google will rate this “Poor.”
(image via https://web.dev/fid/)
FID can be hard to measure manually because of how quickly it typically happens, but you can use Google’s Lighthouse performance tool’s “Total Blocking Time” as a proxy for it.
More detailed information and actionable insights can be found on the official FID Core Web Vitals page by Google. As mentioned, keep in mind that some of these actionable items might require help from your engineering team: https://web.dev/fid/#how-to-improve-fid