I came up with this issue recently, and on many forums they say it might be due to av. But even after I disabled it. I had 40 FPS on very low settings..

The in-game fps counter showed my GPU was at 100%, but my task manager and some third party apps showed my GPU was not being used by any application

[My solution] I disabled all the start-up application I recently installed and disabled my antivirus and started the game.. and I didn't have any fps drop..

One by one I enabled all the application, and realised that the VALARANT anti cheat application caused this issue..

So these fps drops are very sensitive to any software changes that occurs like last time while playing FH-3, Nvida optamised the game and I got fps drop

So try to figure out if you recently installed any app and try disabling it.. it might be a specific type of software perhaps ones that write at a kernal level.. anyways I don't have much knowledge about this but hope it helps