People forget that XP is a seven year old OS, optimised for hardware that old. Vista does a lot more behind the scenes than XP did, and does it in a more advanced way, so to actually equal or nearly equal the performance of XP, it is clearly well optimised. Perhaps you mean the OS should do less and therefore be faster, but I'm not sure how that is progression.
You know Dolph, I first ran XP on an Athlon 750, with 256MB SDRAM, and a 40GB IBM hard drive, and while not instant, it was fairly quick. Not something I can say about Vista and a low-end PC from early 2007...certainly not an unmodified OEM install anyway.
The average user could probably do just fine with Windows 2000, to be honest - their requirements haven't changed a great deal over the years. That is what I resent - OEMs forcing Vista on the general public when they have no use for it.