I meant Intel HT specifically, but I'm going off memory here, and having trouble finding details on those old parts. Agner Fog's current microarchitecture manual doesn't mention HT in its discussion of the P4, but it does include at least one mention of static partitioning of the decoded op queue in the Atom core.

It also describes several instances where Intel's desktop cores used to devote specific resources to each thread on alternating clock cycles, but newer cores have progressively removed those limitations. However, these probably don't quite fit my original assertion because if the OS has literally HALTed one of the virtual CPUs, these alternating clock cycle limitations may have been temporarily removed.

My first experience with HT was on my dual P4 xeons. Performance with HT on was noticeably terrible. It felt like a dog and pony show. It was best to keep HT disabled then. I'm not sure when that changed, but I don't remember what I did on my subsequent Core 2 duo system, but I do have HT enabled on my current (8 or 9 year old) i7 3700 and don't notice any slowdowns. Last I looked, I had to look at very specific benchmarks to find measurable differences. Qualitatively, I don't feel a slow down, either, so I keep it enabled.