It seems to be very common problem. Unfortunately Larian is doing nothing or is unable to repair it. The problem is about that after some time the game puts pressure on only one core of CPU, and therefore even with high end graphic card one will get very poor FPS. If you save game then FPS seems to go back to normal but still it is too much CPU dependent and use only one core and that`s why often drops below 60FPS regardless of graphical settings.
