Wrong. I'm comparing a vanilla 2.1 install with 25 App Store apps installed to a jailbroken 2.1
with the same 25 App Store apps and the jailbroken phone is consistently slower than with vanilla 2.1. Again, that
only difference is that WinterBoard is installed on the jailbroken phone, in addition to the 25 App Store apps.
I guess it's just a matter of opinion, but having to wait anywhere from two to five seconds longer to open apps (including SMS, Photos, Settings, etc) is a pretty big deal to me. The delays add up, and IMO are annoying enough to justify losing the 'cool' factor and fun of customization. I've been jailbreaking since 1.1.1 and have several custom themes (I've made
dozens of custom icons, wallpapers, status bars and dock) so that stuff is really important to me - but having a responsive UI is important, too.