Migration Assistant migrates an item called "Your Other Files and Folders" (or some-such), and a coupl'a years ago when I posted asking if anybody knew precisely what they were, the best answer I got was to look for a log.
Well, it turned out that there is a log, the name of which I've forgotten (but it was readily identifiable), and I'm not sure how persistent it is, but if you go through your Console sidebar your may find it.
When I looked through mine I found it pretty much unintelligible, but you may be able to glean something from it.
Y'know, I'm beginning to wonder if your problem is that your iMac is so old that the optimization included in OS X 10.6.8 just plain optimized it to death? (How old was it when 10.6.8 was released in July 2011?)
