HD footage on my old MBP would not play at all, HD footage on my Imac would play fine, but when compositing HD footage with Conduit, feedback was not instantaneous when inputting values and playback could only happen with rendering; HD footage and HD footage with green screen played back perfectly on my MacPro.

I had thought these differences were due to memory and CPU, but they may be due to the video card, as the NLE in each case was the same (FCPS2)