Kovid, thanks for the info! I did more testing (disabling antivirus, etc.) -- and it turns out there was indeed a small but important difference in the job log output I hadn't noticed. I just saw that the hex addresses of the output profile were different, and didn't notice that, in fact, the output profile name itself was different.
The upshot is that on the 7x slower cases, it was using an output profile of ipad3. Using the default output profile -- 7x faster!
(Just documenting this for others who might stumble on the issue. I played around with some other output profiles, and found some had no impact on speed, some others had the massive slowdown [such as ipad, nook HD+]. I looked at both the "ipad3" and "default" resulting .mobi output on an ipad, and saw no visual difference, in text or images, at least to my eye, meaning I personally don't see any benefit from the 7x slowness, so won't be using that.)
Thanks everyone for all the help. Hopefully this info can help others avoid possibly needlessly slow conversions.
