My PC barely keeps the framerates OK to comfortably fly. Well most of the time its OK, abt. 15 to 25FPS and a little lower when I come to a larger airport.
The most unusual thing is that I have some 3rd party airports with complex scenery and my framerates are OK, but when I come to the DEFAULT FS2002 FRANKFURT MAIN (EDDF) airport my FPS drops considerably. Wierd when considering that FPS are OK with complex 3rd party scenery!

Do you have any added AI? A complex scennery airport with low AI can be ok for your framerate whilst a default scennery airport with very heavy AI can really kill your framerate. Also check what kind of airplanes are oarts of the AI. With my last phrase I mean a high level detaile textured airplane when used as AI can have decrease the performance dramatically. Usually AI addons (like Project AI) use airplanes particularly designed for this purpose.