I don't think upgrading was the reason for Vista performance. MS wasn't in the hardware business back then (and is just a marginal player even today).

They WAY overreached in their goals with Longhorn. When they finally decided to cut back features to something actually attainable, they didn't have enough time to make a high-performance OS.

Windows 7 was a well-loved rebrand of what was essentially just a Windows Vista service pack and improved performance (though it was still too heavy for a lot of the older machines people tried to upgrade to Vista). If they'd have cut back on their goals earlier, Windows 7 is likely a lot closer to what would have shipped as Vista.