[chrisseaton]

In case other people don't see why this would be the case, hyperthreads are good for running a second thread while the first one stalls while waiting for something such as a memory read. Compilers often work with indirect data structures such as irregular graphs and symbol tables which don't cache perfectly and may cause memory stalls.

[aarongolliver]

It mattered more with spinning disks, you could basically pipeline disk reads with optimization passes.