Number of processors used during compilation

Hyperthreading relies on a thread being available that isn't using part of the processor that is used by another thread. For example one thread doing mostly integer math can hyperthread with a thread doing mostly floating point. When you have lots of threads that all use the same kinds of processor resources, hyperthreading doesn't help. It may actually hurt throughput. The Altera devs probably knew that in parallel compiles, hyperthreading wouldn't help.