Recently I upgraded my iPhone firmware to 3.1.3. Immediately after the update, text messaging was SUPER SLOW and the phone was just plain laggy. Opening apps would sometimes result in a white blank screen for seconds and all button presses lagged by 2-3 seconds. I tried your standard soft-reboot (hold down SLEEP and slide to turn off), but that certainly didn’t fix it.
After posting a quick question on the Apple forums, the fix was easy:
Perform a HARD RESET by pressing HOME + SLEEP until you see the Apple logo. Apparently this actually reloads OS X on the phone and it fixed my issue!