[2023] CK3 Masive Render Latency and Sudden FPS Drops

That sounds like an overheating problem. 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, 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


Please report the temperatures of your CPU and video (GPU)
1/ before starting the game up and
2/ after say 30 minutes' play.
If your PC's vendor has given you no way of reading temperatures, install
SPEEDFAN http://www.almico.com/sfdownload.php  

That does explain your weird symptoms ... good luck with that! I hope it doesn't come out too expensive!  

Well, that's not impossible either, although that would be an unusual effect of malware. You could do a deep scan of the system with, I'd suggest, at least 3 different AV apps. Malwarebytes and Spybot are two good free one.  

Here is the clean reinstall process for this game:

- 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

In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.

Good luck!