My main computer is a 3½ year old Dell laptop (2.5Ghz C2D, 4GB RAM) and Windows 8 boots and runs a little smoother than Windows 7 does. Not enough for me to stick with 8 though, I am quite happy to keep it restricted to a virtual machine.

I've had Windows 7 running at peak performance for years in boot-camp, on my early '08 Macbook Pro.

My understanding is that any general-purpose desktop OS is susceptible to slow-down over time unless you properly maintain it. In the case of OS X, it was slow from the beginning (at least, relatively...compared to Windows, on the same exact hardware).