If you're still a loyal iPhone 3G owner and upgraded to iOS 4, you've probably noticed one major new thing: slowness. You can downgrade to 3.1.3, but more than one Gizmodo commenter suggests a fix that's worked for them.
Commenter acidrain69 (oh, youth) notes in a post about Apple officially investigating the iPhone 3G/iOS 4 slowdown that Spotlight search, the omni-search box to the left of your main home screen, is a major culprit of app-switching lag. Here's the fix:
Go to Settings- General -Home - Search Results, and shut off everything you don't need (I disabled Music, Podcasts, Video and Audiobooks; leaving Contacts, apps, Notes, Mail and Calendar checked). Made my phone a lot quicker, more like it was with v2 (I'm still running v3.1.2, jailbroken).
Before, there was a good 2-4 second lag when exiting or loading an app. Now it's back to it's old speediness.
Someone on the WSJ forum mentioned a bug in spotlight, but didn't say how to disable spotlight search. I don't see an option to disable it entirely.
We don't have an iPhone 3G running iOS 4 to test this solution on at the moment, but given the consensus of 3G owners and the seemingly slim trade-off of giving up some Spotlight functionality, it seems like a worthy workaround.