I see this question asked over and over again in forums: "What can I do to increase my FSX performance"? More often than not, the question is answered in ways that I'd describe as... entertaining, but far from true.
Here's a small tip that will fix your FSX performance, almost doubling your FPS, if you are unlucky enough not to have been aware of the consequences:
If you run FSX in Windowed Mode and have the FSX window maximized, sometimes the Windows Start button will overlap your FSX window. If your machine is powerful enough, you'll tend to be annoyed by the seemingly low FPS, but performance will be enough for you to simply be puzzled.
Resize the FSX window to allow the Start button NOT to overlap (or simply hide the taskbar) and you'll find that performance will increase - sometimes even double.
Same holds true for ANY external window that overlaps the FSX display - don't let any of those small popups come in view, as your FPS will drop to almost half (the technical reasons are outside the scope of my note here).
Let me know if you had this issue and if my advice above fixed it!