First Input Delay (FID) measures the time from when a user first interacts with your 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 actually able to respond to that interaction.

As developers who write code that responds to events, we often assume our code is going to be run immediately—as soon as the event happens. But as users, we've all frequently experienced the opposite—we've loaded a web page on our phone, tried to interact with it, and then been frustrated when nothing happened.

https://developers.google.com/web/updates/2018/05/first-input-delay

Even though we don't all agree that it would give us anything, I think it would be at least be cool to try it out.

There's been tweets saying that the Chrome team spent years of evaluating metrics and found that FID most correlates to that "page loaded" - however I haven't been able to find any numbers/stats from them, so it could be the same thing as RAIL.