Actually, XP IS slower than other OS's, and Vista isn't any better. With the ultra high end CPU's you won't notice a difference .... if you have only experienced Windoze on a PC. Take the exact same computer running Windoze XP or Vista and install a Linux kernel with a GUI (Ubuntu is the best all-in-one) and you WILL notice a huge difference. Also you did point out the biggest problem, it's 'tweak heavy', in order to get decent performance you have to adjust settings for almost every new major app, then the 'updates' slow it down even more and use up memory (though mostly during boot time) since most have to be loaded as separate processes instead of being fully integrated. I never recommend Windoze XP or Vista to first time or novice users, I do recommend Ubuntu or buy a Mac. For experienced computer geeks like me we just find what works and often it's just personal preference.
I LOVE Ubuntu (it was faster to install than getting all the Linux programs from various sources). One-touch search and install for almost all free programs. Every utility you could ever need. And all the programs integrate seemlessly and without any extra tweaks or maintainance needed. For Intell PC's I always recommend Ubuntu even for novice computer users (there is also two huge sources of help if you need it online).
If you are not like me (I love piecing computer parts together and fully customizing the hardware) then get a Mac. It's more user friendly than Windoze now.
A side note of humor:
YouTube - Novell Linux, Mac, PC
YouTube - Mac, PC & Linux - Novell Commercial #2
YouTube - MAC & PC; LINUX is Ready!!!