It seems like Necrovision behaves differently then other games, when running with poor FPS. It seems like the whole game somehow slow itself down, when having poor FPS. Therefore it doesn`t seem so much choppy as other games, while having low FPS..
I think fraps is accurate..
Game is pretty HW demanding, when all details are maxed and when using higher resolutions.
Just try lowering your details and see if you have better feeling when having , lets say, 40-60 FPS.