"Cripes, the Mac OS X version of After Effects is absolutely smoked again, and the results are slightly worse than last time in places. Either Adobe isn't tuning After Effects on the Mac at all, or tuning the buhjeezus out of the Windows versions. Hell, even single process rendering on Vista generally spanks multiple processes on Leopard, for the love of Pete.
Vista pwnage aside, it's interesting to see how enabling multiprocessing on either platform doesn't necessarily translate to better performance. The time savings bear themselves out on longer renders, but it isn't always a slam dunk to soak up all your system resources by enabling multiprocessing. If your system is beefy enough, it may be a better bet to give AE a single process and then work on something else while the render happens.
In any event, it's clear from these tests that Vista x64 offers significant "pound-for-pound" time savings for your After Effects renders. We're still in disappointing territory for Mac users, to be sure, but I'm actually surprised this time. I figured the situation would be much improved this time around, and, well, not so much. We'll just have to see how the CS5/Snow Leopard/Windows 7 comparison goes."