Win 8 RTM on my old Q6600 was very fast; significantly faster than any prior version of Windows on the same hardware.
And, not to fan the flames, but Vista was also a good performer on that same system... my non scientific perception was that Vista performance was actually better than XP. Any potential improvement in Windows 7 (over Vista) was less obvious to me in actual use.
Mind you, I'm referring to a clean install of Vista, not a preinstalled boxmaker's bloatware version!
But yeah, Windows 8 is blazingly fast, at least in my usage.