There's also some funkiness around the CPU cache, at least at one stage. If your two HT threads are working on the same data, there was a chance you'd get some great cache performance out of it. However the hyperthread when faced with a cache miss, can cause the cache to get evicted to be replaced with the data it needs. Under those circumstances, performance takes quite a nose dive as both threads are stomping over each other somewhat.

Hyperthreading can be a real mixed bag for performance, though generally good and a lot of engineering effort has gone in to making it shine. As ever it's strongly advisable that people benchmark real world conditions on a server, and it's worth giving a shot with hyperthreading turned on and off.