Has this always happened with this game? Or it was okay up to some point?
Any mods involved?
If not, in the launcher-Game Settings menu choose display mode fullScreen, Vsync off, refresh rate 60.
IF that doesn'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