in FSX you gotta go to settings > then customize > then change the target frame rate to somewhere near your current FPS, then it will take less CPU resources to do, which in turn will give you more FPS.
I did this and i was getting 10-20 fps, I changed the target FPS to 40 and I'm getting 30's now.
just play with the settings, Like the aircraft settings....do you really need them that high when the majority of the time you're in the 3D ****pit and you don't even see the plane?
Just play with the settings.