v12 is as fast or faster in pretty much all areas. (startup, library view switching, etc.)
About the only area where MC12 will not perform better than (or at least as well as) MC11 on an older machine is Theater View, the "10-foot" HTPC interface. Theater View has been changed in MC12 and now uses 3D features and acceleration, and requires a modernish DirectX9 capable video card for acceptable performance.
Conveniently, you can have both MC11 and MC12 installed side-by-side, and there is a 30-day free trial of the new version. I'd recommend that you try it out. Unless you make very heavy use of Theater View (and your video card can't hack it), I doubt you'll be disappointed with the performance of the new version.
