Here’s a quick tip when working with Aperture on slower computers (at least on something like mine, a first generation unibody MacBook).
Even though Aperture works fairly fast even on my computer and I can pretty quickly do my edits, dust brushing usually was slow.
Since I have no significantly structured workflow and I usually did the dust brushing somewhere along the line in my processing and then the dust-brushing tended to be slow. When dust-brushing a so far un-edited image though, the speed improvement is more that significant. It seems therefore to be useful to get rid of the dust first and then proceed with the rest of edits. Easy enough.
But when I already have an image with multiple adjustments and I forgot to do the dust brushing, I can quickly create a “New Version from Master” and do my dust brushing on that new version. Once I’m done, I lift the adjustments and select only the “retouches” and stamp them on my “original” version of the image. Once I’m done, I delete the version, or stamp the dust-brushing on similar versions of the image to save some time.
As of this moment I can not confirm that this procedure is relevant for all installs, but at least it works for me and makes my processing a lot faster.