The thing that bugs me is not speed, but support. MS still supports XP (SP3) today and a couple more years. Apple seems to dump their softwares fast. :/
Actually, let's look at that, shall we?
XP went out of 'mainstream support' in April 2009. It's currently in 'extended' support for commercial (read: paying) customers. Are you paying Microsoft to support your XP system? Are you paying Apple to support your 10.4 system?
Furthermore, XP was available at retail until June 2008 - just 10 months before mainstream support expired. That's not so great. Even worse is the fact that XP was available to OEMs until October 2010, some 18 months after mainstream support expired. That's a winning combination if ever I heard one.
Yes, XP came out in 2001, so you could argue that it had 7 years of support, but it's not Apple's fault that there was a 6 year gap between XP and Vista, nor that Vista had such a poor uptake.
In fact, if you look at the official policy, Microsoft provide mainstream support for '2 years after the sucessor product is released'. Given that Apple, historically, have released OS updates every 18 months or so (give or take), and that they support the current and previous OS releases, you can see Apple have an 18 month policy while Microsoft has a 2 year policy. Not all that great a difference in the overall scheme of things.
http://windows.microsoft.com/en-us/windows/products/lifecycle