That is not true. Every version of Mac OS X for 10 years now was faster on the same hardware than its predecessor. Windows 8 is not even the first Windows to do that — Windows 7 shrank to match the tiny, underpowered machines that Windows ships on today now that the Mac has the whole high-end.
OS X's first four iterations were faster because of improvements in the code. That ended with Leopard, which ran like a dog on older PPC hardware. And from Snow Leopard to Mountain Lion, OSX hasn't really been increasingly faster. Apple has been cutting code from the last three releases, and that speeds things up in some cases, but that's not because the code is improving, but because Apple has stopped supporting legacy hardware, as well as adopting a 64 bit only approach. If your Mac is five years old (or older), you're pretty much out of luck if you want to run Mountain Lion. It's not that 10.8 is slow on those machines, its that Apple prevents you from even trying to install on them. Maintaining speed and stability is easy if you force customers to use only recent hardware from a very narrow list. Windows 8, on the other hand, will run on a 10+ year old P4 computer as long as the graphics card meets the minimum requirements (and runs surprisingly well). I'm a Mac user, but when making arguments comparing Windows to OS X, be honest and include the point that Microsoft is much more supportive of a much wider range of hardware. The parent poster was wrong about 8 being "one of the first", but it's certainly one of the first Microsoft OS's to be faster than a previous version on similar hardware. As far as 7 goes, only the 32 bit version was faster than Vista's 32 version, but that's largely because Vista 32 was an unholy mess. Vista 64 performed well out of the gate, and 7's 64 bit version wasn't really an improvement, performance-wise.