about 1 year ago - AndrewT - Direct link
Please do a full **clean** re-install:

- move any valued save games elsewhere
- "uninstall" in Steam-CK3
- manually delete both the Steam/SteamApps/common/CK3 AND Documents/Paradox Interactive/CK3 folders
- re-install game in Steam, run a Steam Verify when done.
- add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test with a new campaign

In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.  
about 1 year ago - AndrewT - Direct link
Make sure the new ck3.exe is in the exceptions lists of your antivirus app.

Right click on ck3.exe , properties, compatibility. Tick "disable fullScreen optimisations" untick every other box in that dialog, Apply and exit.

Then in the launcher Game Settings menu, select fullScreen display mode, Vsync off, cap refresh rate at 60.

Any better?