Just don't try running traffic at 100%
depending on the size of the airport, running at 100% can be absolutely fine. i do it most of the time and frames are reasonable. i used to run 100% traffic at the big airports as well, the framerate wasn't exactly the greates but at the time i didn't care as long as i had millions of ai planes around me

oh and of course it always depends on your system and settings. if you have a very high end pc, it shouldn't be a problem to run at 100%

EDIT:
My system run FS2004 on low-medium settings with about 20FPS is that enough?
sorry, just saw that, maybe i should read the posts better

. yeah i guess the answer is "just don't try running at 100% traffic"
