I stumbled across this link that will either fix your fsx.cfg file for best performance based on your computer automatically or it will suggest what changes you should make to it. I tried it myself and noticed a considerable difference. I didn't do a before/after framerate test, but I could visually tell a difference and since it's free there's nothing to lose! (I did chose to donate however...)

Enjoy!

http://www.venetubo.com/fsx.html