Number of processors used during compilation

Hyperthreading actually relies on one thread being 'stalled' (eg waiting for memory or a pipeline flush).

Stalls waiting for the floating point unit may be ok, but it would only apply to trig functions using the x87 not to add/sub/mult/divide.

The real problem is that is you have 2 active threads you want them to run on different real processors, not share a single cpu.