23 days ago - AndrewT - Direct link
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.

If those don't help:
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 , exceptions.log  
22 days ago - AndrewT - Direct link
Did you set the cap to 60, as I suggested? If not please try that.

Either way what fps do you see in the game now?

The other issue is you are critically short of free space on both your drives. In particular there is not enough space on C: for Windows to do any more swapping out. Windows' boot drive should have at least 10% free space. In your situation I'd see if you can get like 50gb free, then up your paging file max size to say 32gb

Optional: Increasing the paging file size on Windows computers

On computers that have 8 GB of RAM, the suggested paging file size is 16 GB. www.ibm.com
At the very least unload all other apps before starting this game.  
20 days ago - AndrewT - Direct link
You could be running low on available memory. Did you expand the paging file as I suggested?

Also unload every other app you can before starting the game.