I decided to take iOS4 for the iPhone for a test drive on my 3G. I had it installed for nearly two days before I wanted to roll back to 3.1.3 due to “lack of speed” issues. iOS4 runs slowly on the 3G. Messaging takes nearly 5 seconds to open, email the same, and it took a while getting the phone from a locked position into the springboard.
I rolled the firmware back with the aid of this guide after I got into a recovery loop.
HOW TO: Downgrade iPhone OS 4.0 To 3.1.3 [ Mac and Windows ] on Funky Space Monkey.