Your problem sounds like the "real" and "virtual" CPUs (which make up the two processors in an HT system) are competing for system cache. With a dual cpu (or dual core) each processor has its own cache, but with Hyper-Threading the "real" and "virtual" CPU's have to share the same cache. If the real and virtual CPUs need different data then the HT processor can spend a lot of time moving data in and out of cache (depending on cache size) rather than doing any actual work, which results in longer render times or apparant lock ups. This is one of the reasons why turning off HT can sometimes improve performance.