The login screen with the animated background constantly consumes 100% CPU of 1 of my 16 logical cores. That slows down the browser enough that typing the user name and password has a very notable delay between pressing a key and actually seeing a character on the screen.

The CPU usage is stopped when that's not the active window, so please be aware when debugging or measuring.

That's on Firefox 112, Windows 10, with 4K resolution.