Here's what happens when LR is building the 1:1 previews:
Jim, I'd be interested to see what your CPU usage charts look like with HyperThreading disabled, or with LR set to use only the 12 physical cores in your machine. (You can set the latter by right-clicking lightroom.exe in the Processes tab of Task Manager, selecting "Set Affinity", and then unchecking all of the odd-numbered CPUs, leaving the even-numbered ones checked. The even-numbered CPUs represent the physical cores.)
I've been running similar perf tests on my 4-core machine (8 logical cores with HyperThreading), both for rendering 1:1 previews and for exporting a set of images. In each case, LR4 (and LR5 Beta) uses at least 4 threads to render or export each image. (I suspect it uses each thread to render a different part of the image, with another thread to combine the results into a finished preview or exported image.) When I disable HyperThreading on my machine, all 4 cores have steady peaks at 98-100% utilization for each image, with dips to 25-50%.