A lot of cpu time is done on graphics related things, the GPU is responsible for the final render, but the cpu does things to and for the graphics. For instance, a GPU will render particles of dust, but the cpu creates and places them according to things that happen within the game.
Sometimes the CPU and the GPU will work together, take the same smoke particle and imagine it growing larger as it fades out, the CPU might tell the GPU to move each of the four corner points (vertices) out from the middle (actually it will apply a global scale for speed).
EDIT: PS Scawen: If you haven't already can you please rotate [roll] the smoke particles randomly please, so that dense smoke doesnt look repeated ! I havn't tried in recent patches but last time I put the smoke density up you could see each billboard was oriented the same and it looked quite bad.