[2023] EU4 fps gradually drops from ~140 to ~60

Well, my first thought is that 60 fps is just fine for a non-action game like this!

Or do you see other symptoms of worse performance, such as slow time passing or jerkiness in the maps?

And have you checked CPU and GPU temperatures in the game?  

Just to be clear: you have satisfactory performance in other respects, but would like to see higher fps numbers? That's your goal here?

Does this extend to CK3, Stellaris, HOI4 and CK2 also? Or just EU4?

And, has this always happened since you've had this game/these games? Or it/they were okay up to some point?  

Jiajinlong Kang said: having a 20 FPS when opening a tab is far away from satisfatory

I understand te fps drops in that and other circumstances. What I'm trying to establish is if there are other performance symptoms! So when you open a tab, is that notably slow to happen, quit aside from the fps at that time?

Not that I have any idea of what is causing this on your hardware, which of course ought to be fast!