texace,
Ground Clutter turns off with ctrl-C, (although it makes judging the distance to the hard deck difficult to judge when you're down low). Reducing the distance that the ground is rendered will also UP the framerate, (shift-F1 Full, Shift-F2 Reduced, Shift-F3 minimal).
Dropping the colour depth or resolution may help a little, (starting up AH click on the Video Button).
Final Suggestion: Get a kickass video card. I'm running a PIII 400Mhz with 128Mb RAM and an AGP V3800-TVR Video card, (32Mb TNT2 card), which squeezes 28fps over an airfield and drops to 12fps in heavy smoke, (at 1024x768 res and 32 bit colour).
Spotcha in the Air