Came across this nugget at the weekend and thought I'd share
If your using a Quad Core machine FSX will not automatically use all cores,so you have to add this to the fsx cfg:
Many of you may have known this already(I did). What is important here is the number(14). I had mine set to 15 as advised in some forums and publications. Setting to 15 uses 4 cores so FSX starts on core 1, however setting it to 14 starts FSX on core 2 which means it dosen't have to fight with the operating system and other processes, thus giving it a clear(ish) core to start on. Hope this makes sense. I have seen a good improvement using this.
cfg can be found here on my machine: