[Golden Eagle]

Well his experience, as well as mine, is it doesn't run well on older hardware. Especially not the newer versions, unless you get some stripped down version that might not even include a GUI.

[blowfish]

Well, I have had the opposite experience. Any OS can be brought to it's knees if you run enough stuff on it. KDE is pretty adjustable, so you can enable or disable some of the bells and whistles depending on the horsepower.
That said, there is certainly a lot of work going into continuing to improve the performance of the KDE desktop. That's certainly one advantage Microsoft has: if you can dictate terms to the hardware manufacturers and maintain a monopoly on the UI you can tweak things to your liking.