InfoWorld has a very interesting test between Windows XP, Vista and 7 in terms of multicore performance, and it seems like Windows XP still wins (right now). But there are even more interesting things.
The interesting bit is that, using the testing workloads they came up with, they were able to find that Vista and 7 were almost exactly the same, barring some tweaks, and very different from XP under the hood. Our analogy would be that Windows XP is an orange, and Vista and 7 are a banana and a ripe banana, respectively.
Another point Infoworld brought up is that both Vista and 7 are actually more optimized for multicore performance than XP, naturally.
Taken to its logical conclusion — and disregarding for the moment external factors, like bus speeds, I/O contention, and memory latency — Vista would ultimately overtake XP when the core count reaches between 32 and 64.
And in Windows 7, they suggest that it will overtake Windows XP at around 16 or 24 cores. So although it may be slower now, in a few years, when you have 16 or 24 cores, you're going to be thankful for Microsoft for their work. But until then, Ballmer can expect lots of angry fist shaking. [Infoworld via Slashdot]