Cloudflare says: First Input Delay [FIC] is a way to measure interactivity. FID measures the time that passes between a user's first attempt to interact with a webpage and when the webpage actually responds; that is, when the browser is able to carry out the requested action. This measures the user experience of when someone can click on the screen and have something happen. Website owners should aim for an FID of 100 milliseconds or less.
