First Input Delay
developers.google.com
developers.google.com
The screen may appear and the mouse may become responsive: that's time to interaction.
The box input will still be inactive and/or touchid will not yet work: time to first input.
It's very important to know when or how users are hindered from/during using your product
Back in the days (90s, begin of 00s), Windows would boot up, but it wouldn't be usable right away since various of its services and programs were still starting up.
Once it was started up, MSIE would launch very quick. Quicker than Netscape or Mozilla indeed. That's because MSIE was pretty much already running when Windows got started.