Finally tracked the problem down!
Idiotic as it may be it was a windows set to "power saving" mode that kept Qimage running at 800mhz on a single core. Benchmarks taxed the system enough to kick in all 4 cores at 3.5ghz, but qimage by itself wouldn't do it... Never occurred to me to check as I never touch power management on my desktop, and my ASUS control panel is always set to "performance". Simply switching windows to "performance" lets me hit 25-40% CPU load at 4ghz in qimage: 5x-10x faster depending on job.
Thanks again for the help!
Terry