Several iPhone 3G users have experienced a significant slowdown, including slow app launching, delayed reactions to taps, and generally poor interface responsiveness, after the iOS 4.0.x update.
We previously noted a few fixes for this issue (performing a hard reset, restoring but not from backup and closing extraneous Safari windows) with which users have experienced some success.
Users have now reported that the following procedures can also provide a significant speed boost. Unfortunately, they also disable key functionality:
Turn off Spotlight. Navigate to Settings -> General -> Spotlight Search and turn off all the options by tapping each one. Next, perform a hard reset by holding down the sleep/wake and home buttons simultaneously for roughly 15-20 seconds, until the screen powers off then an Apple logo appears, which signifies a reboot.
You can experiment with turning off only some options, but most users only report a meaningful speed boost if all options are turned off.
Turn off MMS. Navigate to Settings > Messages > MMS messaging and slide MMS Messaging to “Off,” then perform a hard reset as described above.