andrew_the_cat said: Just tried rolling back to older versions, they still run badly, even though they ran fine earlier in 2021.That pretty clearly shows this is some local issue in your machine, rather than a game problem as such.
Does that also affect say CK2, EU4, HOI4 or Stellaris?
DXDIAG is a program you run from the Windows search box on the task bar. After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach here your Documents/Paradox Interactive/CK3/pdx_settings.txt .
From your Documents/Paradox Interactive/CK3/logs/ folder, attach here system.log , error.log .
Please report the temperatures of your CPU and video (GPU)
1/ before starting the game up and
2/ after say 10 minutes' play.
If your PC's vendor has given you no way of reading temperatures, install