You might want to try asking this question in the Technical Support forum as I bet more people in there can help.
Typically, I think you could play with 25 frames/sec IF that is with a reasonable number of aircraft around. Smoke/Clouds seem to be the biggest thing that impacts frames rates, though lots of airplanes, or ones with lots of shapes (Bombers) can also be a serious problem. You can use CTRL-I to see what your current frame-rate is. Once you are in the teens with frame-rates you are typically having problems.
To improve framerates most people will recommend better hardware (memory, CPU, especially videocard), which is obvious. You can also turn down the detail level a bit (SHIFT-F1, SHIFT-F2, SHIFT-F3 will change that I think off the top of my head). There could also be some tuning stuff you can try but I think the Technical Forum people could help you more than me. I've also heard that matching your desktop resolution to your game resolution settings can help, though I've never tried that. Not so sure about that last point.
Try the technical forum though, I bet you get lost of info from there.