You're confusing things that aren't related. XP was plenty stable out of the gate, given it was a minor update to Windows 2000, which would have to be pried out of the cold dead fingers of some people if they had their way. The problem was that low end systems, specifically integrated graphics chipsets, at the time simply weren't up to the task of XP's new Luna interface. Same problem hit Vista with Aero. It took a couple of months before Intel and the other low end graphics chip peddlers had something out that could reasonably handle Aero at a price point that would work for cheap sub-$500US systems.
As anyone who studies interface design will tell you, people put huge amounts of stock into the responsiveness of the interface. If the interface is sluggish, then people just assume the computer is slow. We like our instant gratification of some kind of visual feedback to acknowledge that we clicked the mouse button or hit a key on the keyboard. If you have an underpowered graphics card, which is pretty much every integrated graphics solution out there, and it can't keep up with the demands for its time... Well, things seem slow. That, and all too many people buy these cheap low end systems, but expect the same performance out of systems that cost 4X as much. They want prime rib on a dog food budget. Those cheap low end systems are a great deal for the big box retailers like Best Buy, who will then make a fortune selling you all kinds of upgrades to bring the performance up to an acceptable level, not to mention sell you Geek Squad services to put it all in.
By all accounts Microsoft has learned a number of lessons from Vista, and Win 7 will be more of a refinement of Vista. It SHOULD, in theory anyway, run as well or better than Vista on the same hardware. Actual benchmarks so far show no actual improvement in day to day tasks, but Microsoft did kind of "cheat" and boosted the priority of UI related tasks, so the UI seems more "snappy" and responsive. Giving the illusion of increased speed. It will SEEM faster, even though it's really probably no different from Vista in the end.
I know there's always this Luddite notion that goes around about them needing to "work the bugs out of it" but people seem to think that this happens within a couple weeks. That somehow, if they buy a copy of Win7 a month after launch, their install disc will miraculously be different from mine bought at launch. It won't. It's just one of those ideas that sounds good, until you finish the thought, and realize it doesn't make any sense at all.