Looks like it's usually CPU, and the lowest it normally gets is 20fps... except when it decides to go at like 1fps for just long enough to get me killed, of course.
Often I can never manage to put my reticle on a target... I thought it was the choppiness, but now I notice I'm just having trouble seeing where the reticle is... why can't they just put a ****ing crosshair in the center of the screen, instead of these 4 tick marks that are everywhere BUT where I need them to be in order to put them on my target... yeah yeah, I know the separation of the tick marks indicates how much scatter there will be to my shots, but couldn't they have that in addition to a standard crosshair that actually lets me see where I'm aiming? Grrrrr.