I was thinking the same, no great improvements on FPS with old cpu's. Anyway if I can decreased water graphics in theory It should work much better, but a 20% to say something of low FPS aren't so much FPS.
I think I'wont return the game, 16FPS of prepar3d looks much more smoother than 16FPS on FSX.
BTW, bf=0 and affinitymask doesn't give me much better performance maybe 2-3 FPS more... weird...
Daniel