[2021] Huge consistent FPS drop

There are some mods in use there - does this also happen in a new unmodded campaign?

I see you got the new nVidia driver already - did this only happen after that installed?

Did your clean reinstall include making sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again?  

Gidders91 said: The mods have been installed for a while, but I've been playing only iron-man campaigns recently so have disabled them. Should I try removing them completely?

I'm not aware they can have effects when not enabled, but no harm in trying that. Were any of them updated coinciding with the onset of this problem?

Gidders91 said: The issues started happening this week, maybe Monday/Tuesday. From what I recall, the FPS dropped whilst I was in a campaign so I tried restarting and it's never been the same since.

Ok, so it definitely comes from outside the game since the most recent patch was on 16 March.

Gidders91 said: Can't remember exactly when I downloaded the driver though. Should I try reverting back to previous ones?

I haven't noticed other reports of a problem with the current one, but again there's no harm in testing that possibility.