It's the GPU in this case.
As the GPU is not on a card, we can't do much there. The uptick in the CPU is not big enough.
Look at today's 1/12/2012 comments at http://www.cleartechinfo.com/Total-War-Shogun-2-System-Requirements-Cant-play-it-on-your-computer
They are complaining about speed and they have an i7.