I've used Windows for years now, and recently i've been on Ubuntu for about 3 days now, and I seriously find it very slow. Here is my PC spec:
AMD X2 3800
DFI LanParty UT Ultra-D
2x SATA2 RAID 0 WD hard drives
Tagan 530w PSU
1GB ( 4x256 ) Corsair PC3200
Windows will boot in like < 20/30 seconds, yet Ubuntu takes near a minute to boot, then it has to load Gnome which takes around 10 seconds but with Windows it's < 5.
Applications load amazingly slow, with Windows Firefox opens instantly yet with Ubuntu/Linux ( I've tried many distros, all have this problem ) it takes around 10 seconds to load, or sometimes it doesn't even load at all.
Generaly the whole Gnome desktop feels slow and slugish to do anything. Windows is just instant really, Ubuntu/Linux isn't ( even with ATI drivers installed )