Intel Corp (INTC)

[chipguy]

IIRC, it had a negative impact on single-threaded performance.

P4 was a twitchy microarchitecture at the best of times and on
all models of P4 HT was a mixed bag. Unless you were focused
on one workload and it could be empirically shown to get better
throughput with HT the best thing to do on P4 was turn HT off.

In just about any CPU HT/SMT will usually cause a tiny drop in
single thread performance if for no other reason than lower
cache and TLB hit rates due to conflict misses.