I have been running 4.157, and what has been the slow down as far as my machine goes was the Pagefile and physical RAM. I haven't bothered with getting another 8 GB of RAM but I did increase manually the Swap file in windows which gave me a few extra FPS.
This machine is pretty unique, I am running a pentium 3258 OCed to 4.2 ghz. with 8 gigs of RAM and a Radeon 550 video card. With medium settings I am getting between 25-30 FPS. With the Benchmark scenario, 20-40. I am assuming this game, 4.1 is using multiple cores whereas the 4.0 appeared to be using only 2 cores.