You're close... USB was intended to be a cheap alternative to the likes of FireWire, and as such they cut quite a few corners. One of the corners cut involves using the main CPU, not a dedicated control chip, to do most of the heavy lifting on USB. So FW works more or less the same no matter how bogged down the CPU(s) are, while USB will suffer.
And of course theory and reality never quite meet with computers. There is latency involved with converting the signal from the HDD to FW/USB, and then back again, fragmentation on a drive can result in the head having to move more often to find a bit of room instead of laying the data down in one nice smooth continuous track... A lot of people don't catch on to the fact that FW and USB are measured in MegaBITS per second, while HDDs are in MegaBYTES per second, which gives them rather inflated expectations.
In the end, I think you're spending way too much time and effort on a fairly trivial subject, but I suppose a person could waste their time in far worse ways.