My computer also has issues with performance! Unreal!
I own an Asus Geforce GTX560 Ti Top card and even this can struggle!?!?!
Anyway, In the nvidia control panel, I changed the texture quality from High Quality to Quality and the GPU temp went from on average of 80-85c to 60-63c which is great! Also a decent performance boost in most areas.
I also noticed having very high graphical settings in game can cause lag!! So be sure to keep everything set to just high!