Thanks for the quick tip, even before I remembered to mention that I'm running Mixxx 1.10.0 on Xubuntu Oneiric.
I'm generally knowledgable about computing and Linux applications, but I haven't yet tried using a patch. I assume that this is a patch to the source code, requiring recompiling. I will probably stay out of that (I have another note posted regarding a workaround).
If you particularly need people to try the patch, and want to write or point to instructions, I can push myself toward a more sophisticated participation.
I also doubt that your patch affects my problem. It appears that Mixxx is inspecting every sub*directory in my library space, and the disk seeks are probably eating up the time. A faster rescan for me probably depends on a more sophisticated way of avoiding unchanged areas.
My problem suggests the value of a partial rescan feature---point to a subdirectory and rescan only that. I found such a feature on Rhythmbox. There seems to be a workaround to get that result through the File Browser, but I have another problem there, and anyway it's natural for a user to look for partial rescan in a submenu to the rescan function.