When I first launch the game, I can see my GPU usage is at 100% most of the time. I have pretty solid FPS and rarely drop below 50ish in the worst situations.
But after 1-2 hours of playing, my GPU usage drops to 80%. And my framerates drop considerably, as in low 20s sometimes.
I can fix this by quickly restarting the game and have a couple more hours of smooth gameplay.