• The time from when a user first interacts with a site (i.e., when they click a link, tap on a button, or use a custom, JavaScript-powered control) to the time when the browser is able to respond to that interaction. It is the length of time, in milliseconds, between the first user interaction on a web page and the browser’s response to that interaction. Scrolling and zooming are not considered. FID was a Core Web Vital metric. ← MDN Web Docs ↑ web.dev/fid

1. Related terms: Interaction to Next Paint · Web Vitals · Interaction design · Round-Trip Time
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