I know this topic was covered a few years ago but I just recently discovered it and thought to pass it along. In the past I would open up FSX and then bring up the Task Manager (Ctrl+Alt+Delete), click on the process tab, right click on fsx.exe and assign it a High Priority. This action allows FSX to take precedent over all other programs running and afforded a marked improvement in FSX performance. Unfortunately, this had to be repeated every time I opened FSX since there was no way previously you could "save" the priority setting in the Task Manager. Prio, a free program, allows you to set and save your priorities so you do not have to manually set it every time you boot FSX up.
Prio can be found at http://prio.en.softonic.com/