It's quite clear that the processor and memory capacity of the 3G is just not capable of handling the new revision of the OS. I've even heard of the 3GS suffering a small amount from the update. Apple really needs to think things through, and properly test their products BEFORE release. Look what happens when you don't.
My 3G is slower too since OS4. And no you cant safely downgrade. It is possible but you risk bricking your phone, its harder on Windows than Mac to downgrade. I havent tried it since reading the instructions.
I remember when i upgraded to ios 4, i commented that it seemed faster. However that didn't last long! Its slow! Its slow at loading web pages, i've given up on the online games because they don't work and the keyboard can't keep up with me!
I decided to upgrade to the iphone 4 when Apple started advertising it. However I think the performance of my 3G has definitely given me incentive to do it now, and i placed an order online at 12am this morning!
I still have faith that Apple will fix things. Just look how good the iMac is! We all have to remember that they are making leaps and bounds in mobile technology! Maybe they should wait a little longer to test thing better, but it must be hard for a company at the top with the cutting edge tech, that everyone other company looks up to and wishes they had, to make a society of Tech hungry people with an attitude of i want it and i want it now!, to wait.