Hello,
I have used parallel on a MacBook Pro earlier and I didn't felt the compilation to be too long (nothing more than 1mn or so on big projects). Do you have any antivirus installed on the Win image ? Do you have enough physical RAM (is the PC still slow when launching Windows programs) ? Have you set a big paging file ?
If Windows is slow, have you tried to use a program like Process Lasso (evaluation) or Cacheman (evalution) which could handle the processes priorities ?
In fact, the Mac was quicker than the PC I do use now (Intel Celeron N2830).