My mac is running splinter sell REALLY BADLY!!!!! HELP?

I purchased splinter cell conviction on my mac via steam (so its all legal). i have to put all the settings on the lowest possible setting and turn all the other graphical options off, then i have to run it at 640x480. EVEN THEN i only get around 10 fps. Now you'd probably think i have an old 2002 mac laptop but no i have a 2010 model 27" imac i would link you but i believe they've discontinued the model i own. Any ideas would be greatly appreciated. Thanks