I've had all kinds of graphic and slowness problems running VW under parallels. As far as I can tell parallels still adds a lot of overhead and doesn't really have direct access to the video card. Bootcamp on the other hand is fantastic. There are several applications that will not run under parallels that work just great under bootcamp. After a few similar experiences I won't run VW under parallels, but have no problem with it running natively on Mac, Windows, or under bootcamp on the Mac.
This has all been with 2008, but my guess is that 12 would be the same.