I had similar problems as the original post on my iPhone 4S:
- Tapping on a text from the list would be followed by a 3-5 second pause until the text message displayed.
- Interestingly, you could go back and forth between the list and a particular text sender in a fast (normal) manner, but if you choose a different text sender from the list, that annoying 3-5 second pause would come into play.
- Sometimes I couldn't even get to my Messages list and was just presented with a blank new Message text screen.
- Sometimes I couldn't turn iMessages "on" in the Settings page.
- Sometimes only the last text of a sender would load after the delay; but then waiting a bit, past texts would also be displayed.
- When sending a text, sometimes the progress bar would stop near 80% done and not send the text until a minute later.
- Occasionally, people would send me a text and it wouldn't be delivered to my phone (although most texts to my iPhone would register fine).
All of these behaviors seemed to crop up at the same time. I associated them with the 8.2 iOS update.
This didn't solve my problem: Toggling any of the settings in Messages, Shut down/Start up, or Factory Reset (via iTunes) and using Backup Restore.
This did solve my problem: Factory Reset (via iTunes) without Backup Restore.
It is a bit of a pain to reload my apps and other settings... but the Messages section works flawlessly now. The Messages section was so disabled it was essentially unusable. I was "this close" to buying an iPhone 6, thinking that my 4S was just too old to handle iOS 8.2. I would have been wrong. Hope this helps others.