Is there any way to speed up my iPhone 3G since upgrading to iOS4

According to this post on Gizmodo you can edit the preferences for the Spotlight search feature for a speed boost:

Go to Settings->General->Home->Search Results (Settings->General->Spotlight Search on later iOS levels), 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

Reset the phone to factory defaults and reconfigure it. It's a lot of work but the performance is almost as good as iPhone OS 3.1.

I've just tried this yesterday and I'm very happy with the results. If you use GMail and/or Exchange accounts (or others) to manage e-mails, contacts and calendars you'll only have to reconfigure your apps.

Apparently doing a hard reset a couple of times in a row can improve the performance:

Speed Up iPhone 3G + iOS4 with a Hard Reset?

(via Lifehacker)

I'm wondering if some of the apps are holding onto memory when you close them, at least the ones that are utilizing the new multi-tasking functionality, and slowing it down? Try removing all of those from the multi-task tray thingy and see if that helps.

If you jailbreak, you can disable some unnecessary launch daemons...not a trivial task though. An interesting read, if nothing else lol.