After I put iPhone OS 4 onto my 3G, I noticed it was a little slower to respond to my inputs. In fact, with every update leading up to OS4, I had noticed slowdowns. OS4 was not a huge jump in sluggishness, but it was the last straw.
I've been googling the topic every day since, and today I hit paydirt. The solution is to TURN OFF SPOTLIGHT. I never used it anyway.
It is in an odd place: Settings General HOME BUTTON Spotlight search. Unchecked them all.
Happy phone!!
Just in time for my new iPhone 4 to come...