I know this is a old post but I found a rather interesting post which may help.
Try adding this to your fsx.cfg
Code:
[JOBSCHEDULER]
AffinityMask=14
Using 14 will try to set FSX onto all cores except core0 for a quad core system.
1111 1111 = 15 = all cores used (a quad core has 8 cores because of hyperthreading)
1111 1110 = 14 = all cores used except the 1st one