My notebook came with Vista. I upgraded it to Windows 7, and then downgraded it to XP. Here are some measurements I made:
Time to run Firefox: Vista 3.6 s, Win7 2.8 s, XP 1.25 s
Time to copy a 1 GB file to same hard drive: Vista 63 s, Win7 120 s, XP 99 s
Hibernate: Vista 50 s, win7 20s, XP 20 s
wake: Vista 35 s, win 7 41 s, XP 30 s
shut down: Vista 170 s, win 7 62 s, XP 12 s
start up: Vista 4 to 8 minutes, win 7 2 or 3 minutes, XP 43 seconds
(difficult to measure because it is possible to use the computer while startup activities carry on for some time after)
Processes running: Vista 72, win 7 57, XP 18
Threads running: Vista 787, Win 7 607, XP 258
physical memory free: Vista 315 MB, win 7 63 MB, XP 1723 MB
(2 GB installed and that is the maximum for my notebook)
Clearly XP is far superior to Vista and Win 7 for a computer with 2 GB of memory. The one compelling feature of Win 7 is the ability of the 64 bit version to let a program use more than 4 GB of memory. If you don't need that, or you don't have a computer capable of it, then you will be disappointed with the upgrade.
I turned off all the things in Vista that annoy people, so it never bothered me. But it followed the rule that every new version of Windows is bigger and slower than the one before. Windows 7 is faster in some ways, so it is the first exception.
Lots of people are still using Windows 98, so no worries about XP no longer being supported. That just means you won't be annoyed by it constantly downloading stuff and telling you to restart the computer.