Vista is actually a better OS than many gives it credit for. Now, before everyone goes for my throat, I had Vista Ultimate SP2 (through TechNet). Installed it on my desktop, it ran as good as than Windows 7. It was a resource hog, but my PC isn't exactly filled with plenty of ponies under the hood. Windows 8 CP is fixing that.
I installed it in VBox on my notebook, gave it 3GB RAM, used MSE 4 for protection, it ran quite well. So I would say that a lot of the performance hit was due to underpowered computers that it may have been shipped on, as well as heavy AV/IS solutions like Norton was at that time. No wonder so many users reverted back to XP Pro.
My sister had such a computer, a low spec CoreDuo, with 2GB RAM, and Vista 64 bit installed. I increased the RAM to it's max, 4GB, it ran better. Next, I removed Norton IS from her PC (they told her it was the best & she believed it), and installed MSE 2.0. There was an improvement, but not as much as I'd thought.
I bought & installed a OEM copy of Win 7 Pro, it was slightly better, but not worth the $119 that it cost. Only when it was taken to the shop, it was fixed. The man at the computer shop installed a newer CoreDuo (used) for a total of $50. Then it ran fine. Any OS has to have enough power under the hood, or it's going to run slow.
Cat