Ever since I’ve installed iOS4, my iPhone 3G has been quite slow. And there are other users who are experiencing it. Some of them just downgraded their respective phones to iPhone version 3.1.3. Some users have reported that the Spotlight Search is the main suspect on why iPhone 3G users are experiencing this.
Apple will be investigating this problem as reported in WSJ.
I guess Apple will be releasing a software update to fix this issue. For the mean time, why don’t you try the solution posted in LifeHacker. First, you need to disable spotlight searches. Go to Settings > General > Home Button > Spotlight Search. Then, disable all the options there by clicking on each item.
One user said that a double-hard reset (in addition to disabling most of the options in the spotlight search) worked for him. In case you don’t know how to do a hard reset, just press the Home button and the Sleep/Wake button simultaneously. Wait until the Apple logo appears before you release the keys.