1. In Vista, there was a completely new Antivirus framework that AV vendors had to use in order to support Vista. Good idea in theory, but because it was brand new (and because vendors had to write brand new code to support it), it had a fuckton of problems, mostly contributing to an enormous performance degradation compared to XP. Like, horrendous. And since every laptop you could buy in the store had 3rd party AV because that's how OEMs make $$, whoops every laptop is absolute garbage out of the box.
2. Windows Vista completely rewrote the USB stack, an extremely complicated, timing-sensitive component (like, add a printf, break a device kind of sensitive), and about 80% of the way through, the lead architect who planned the rewrite left the company. Meanwhile, there were a _ton_ of devices which weren't actually correct wrt the USB spec, but just happened to work with the timings that WinXP solely by accident would exhibit. A friend of mine made a lot of money in bonuses by slogging through bug after bug in the USB stack trying to make it right for Vista SP1.
It's funny, because Vista shipped a ton of great features, that then got attributed to Win7 - it really set the foundation for the modern Windows OS until Win10 came out.
So, if you learn anything from the story of Windows Vista, it's this: Performance is not a Feature, but it sure can make all of your other features not matter at all