Shift uses PhysX. If You have Nvidia card You can run PhysX on GPU, with ATI card You must run PhysX on CPU and it puts too much load on CPU if You have dual core.
I built budget PC for a friend not so long ago with E5300 @3.2GHz and HD4850 and was surprised to find out NFS Shift lagging. Swapped in my GTS 8800 and it was runnning fine. Tried his HD 4850 in my PC with Q6600 @3.2GHz and that was running fine as well because quad had the power to run PhysX on CPU.