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