Originally Posted by
wotsit1982 Erase and restore has failed to fix the problem.
Screen freezes continue in Settings and Messages apps.
Some message threads have some history - most only showing last message. Screen freezes for about a minute when trying to load each message thread.
What a disaster.

Then something else is wrong with your iPhone 5. My iPhone 5 that my youngest son now uses runs well with iOS 8.3, and I updated it OTA. If you did a clean install and then set up your device as new, and not from a backup, then your device should run quite smoothly and not in the manner you're depicting. I recommend that you not go to Settings - General - Reset and then do an Erase All Contents and Settings, but put the iPhone in DFU mode and then restore it via iTunes via a computer.