I tried the game on two different but both high end PCs(fast quadcore and a dualcore, graphic-cards with 512 and 1000 MB)and I really could not play it! Diffuse or lagging graphics, stops when accessing a map or the inventory,bad level design.I felt being in a pacman game. The only really good feature was the sound,-good dialogues! It is a pity: the story is interesting I should have liked to play but ,-...back to Gothic or the Witcher(what a difference!)
I'm playing this on a 3 years old PC and have no slowdown - not even in Venice and all options are at max and resolution at 1920.
Maybe it's because I run the game from a separate hard disc. It seems pretty load-intensive to me. You could also try to set "maximum pre-rendered frames" to 0 in the nvidia control panel... no idea what that option is called in the catalyst or if it's even there.