It's pretty marginal, but consider getting hold of the plug 'disable smoke trails'. This should make things run quite a bit more smoothly if the Voinians and UE Navy are slugging it out where you're trying to fly.
v1.0.1 didn't have smoke trails, but did include a few bug fixes that are worth having. I'd go for that if v1.0.2 is unacceptable. Where you'd get it nowadays, though... hmm. Somebody here must still have a copy.
Use 2x speed (caps lock) only when on long flights in safe areas. Keep the function in reserve for that. As for increasing speed on the options slider, it's not a terribly bad thing to do, since it has a side-effect of increasing the range of your guns... but you're really only changing the frame rate; combat against Azdaras and other fast fighters could become a bit of a nightmare.
Uh... otherwise, you seem to be doing all the right things to speed up your Mac. Try changing screen resolution to 640x400 maybe? Assuming you're playing at 800x600... which is easier because you can see hostiles from further away, but must require some effort on the part of the '040.
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