Hello.
Looks like the game might have a memory leak somewhere as I've noticed the FPS dropping about after a few hours of playing continously.

Well, did you check and confirm anything unusual about RAM/ VRAM usage?

I have this memory leak too, and the game's performance is worse. But it's also normal, I think. Well, maybe 6GB of RAM and 2GB of VRAM is not enough for playing at lowest settings.