Hi,
I'm having the same problem, also on a rMBP with 16Gb of RAM.
FYI, I am running XP with 2Gb of RAM allocated.
The VM is performing terribly, unless I turn off folder sharing with the Mac. As soon as I do that everything starts working fine.
I don't have a solution so far, but will post anything I find.
BTW, my rMBP is running Mountain Lion 10.8.2.
Regards
Darren.