After some research I came across this interesting article, written in 2009.

http://www.pcgameshardware.de/Empire...rmance-678262/

In this article 14 CPUs from 2009 are tested and the results can be summarized as the higher the clock speed and the higher cache memory levels 2 and 3 is, the higher fps can be achieved in Empire Total War. The article suggests that only 2 cores are used in this game: triple or quad core CPUs are only slightly faster than models with two cores.

Hope this helps for those like me who are studying carefully how to improve this game performance and to understand the hardware limitations.