CAD is very difficult to parallelize. Think about the parent/children
relations in your feature tree, and you'll see that 2 or 4 cores are
of little help to rebuild feature quicker.
Some specific features which are fairly long to rebuild might be
partly parallelized. I'm thinking of shells, which could be possibly
divided into 2-4 parts each calculated on a different core, then merge
the results (which takes some extra time compared to a mono-thread
calculation). I'm fairly sure Parasolid works on such things, because
it's their business rather than SolidWorks. The day Parasolid (or
ACIS) will be multithread, all the CADs will take advantage of it.
AFAIK, at this point SolidWorks uses 2 cores only in drawings, where
the high-quality for printing is computedin the backgroud while you
work on a low-quality drawing on screen. Photoworks and Cosmos are
multithreaded, since parallelism is fairly straightforward for those