I'm seriously not understanding why so many people are having performance issues with the 3g and iOS4. I'm not doubting you guys, but seriously, I've had 4.0 on my 3g since the GM was released and it's running probably 25% FASTER than 3.0 did.
I don't know what the trigger is, but yes the performance is horrible on mine (too many apps, too many mail accounts, the apps I run???) I've used every beta version released and it has improved, but even the final is dog slow.
Typing text and emails I get 5-7 words ahead of it without trying hard.
I'm getting a iPhone 4 Thursday, but plan on still using the 3G as a touch after. Then I'll back off the number of apps and mail accounts to see if that helps.