This is very interesting, all these 2G and 3G users claiming that 3.0 performs worse for them than 2.2.1 did.
I am exactly the opposite. I loved 2.2.1, and was afraid that 3.0 was going to add back a bunch of bloat again and slow down my phone. After all the many disappointments that had been handed to us, 2.2.1 was a breath of fresh air. Safari FINALLY felt stable, and that OS was a steady workhorse for me for 6 months.
3.0 is FASTER on my 3G, and this is on a jailbroken device, too. I was extremely pleased with the performance of 2.2.1, but 3.0 is snappier in all respects. Furthermore, it is quite obvious that Apple put the OS on a RAM diet (which it severely needed). The other day I was browsing around with Safari while also listening to Pandora and signed into Nimbuzz (thanks to Backgrounder), which is a feat that I NEVER dared try with 2.x of any variety...Safari would cause the backgrounder applications to crash out in no time. And I regularly see 42MB+ of free RAM on bootup, which I never got close to with 2.2.1.
There is only one area of 3.0 on the 3G where I can honestly say I notice lag that is somewhat annoying: when CLOSING, not opening, apps. Sometimes it's fine, but I would say that 1 out of 2 times the app will not close out immediately when I press the home button. Somewhat annoying, but liveable. I have a friend who also has a 3G running 3.0 and has the same issue, so I just figured that it was a common 3.0 glitch that has probably been fixed on 3.1 already.
Anyway, that's my $0.02. Oh, and BTW, I just typed out this whole post on my iPhone! (would never dare try that with 2.0/1/2; Safari would have crashed by now)