I don't know how much this contributes to your problem, but I do see one CPU-eater: when you save as QuickTime, saving each frame takes successively longer, until it's really dragging along. I've seen it even with low resolutions like yours (although I have a much slower machine). For me the solution was to render to sequenced TIFFs and then re-save the finished animation as QuickTime. One other thing: if you're using blurry reflections those can also be a major CPU hit.