[2021] Linux. CPU usage goes to 100% in menu and fps is 4.

I see nothing in there to explain this. While we work on it keep an eye on the YOU'VE PLAYED time on Steam, if it approaches their limit of 2 hours STOP and seek a refund from them, as they will not honour that after that time.

in Steam please click on Steam-Help-System Info, and paste everything there into a text file and attach that here.

I assume CK2 runs okay for you?  

That could happen to any of us so I guess we're all idiots

All the best!