Same situation on my 3g. Apple probably shouldn't have even pushed this update to 3g models, to be honest.

It made my 3G nearly unusable too, sometimes taking a minute to open the SMS app, or contacts. Typing was very difficult because it would hang once pressing a character.

3G is unusable with iOS4 for me too. Only way to keep using it is to close all open Safari tabs and occasionally do the following: hold sleep button until power off screen (swipe) then hold home button until home screen returns.

I have also experienced extreme slowness with ios 4 on 3G. Clearing the safari cache and restarting made things quite a bit faster - but still slower than ios 3.