I'm not in front of combustion right now to my terminoligy may be off.
When working on particles I've noticed that if you click on the corresponding footage control and set the output or resolution to the lowest (eighth) the previewed playback speed increases dramatically. I know that sounds obvious but the particle preview'd playback still renders at full openGL resolution, only almost in real time.
Obviously you couldn't render at this res as it's far too low but for previewing you motion on a path etc it's ideal.
Actually now that I think about it this may also give better playback when previewing other vector or openGL accellerated layers... I'll give that one a try when I get home
hope this helps