I had a weird bug where the game didn't clear itself from memory after quitting to desktop. It doesn't show up in task manager as being open or anything, but it still eats GBs of memory. Also, the GOG launcher thinks the game is still open too because my play time kept going up overnight when my computer was asleep. The only way to fix it and clear the memory usage is to restart the computer. Please CDPR look into this, this looks like a serious memory leak that could be affecting in-game performance as well.

That is a windows 10 issue, not a game specific issue. It happens intermittantly with any kind of program, not just games.

If your keeping an eye on your free mem, your a step ahead of most.

I would reccomend you get Procexp (Process Explorer) and use it instead of the built in task manager.

You can search "procexp" yourself or follow my link. It's part of the sysinternals suite written by microsoft. However you find it, be certain you are downloading it from Microsoft and not some third party.

https://docs.microsoft.com/en-us/sysinternals/downloads/process-explorer

If on windows 10 try this

Windows Key + Ctrl + Shift +B

Restarts the Gpu driver then check your running processes, If it's still not terminated could be another thing holding it back like a AV or Malware scanner or even the opposite of those.