The 3.1 started to become incredibly slow once I installed at the latest updates. If it’s factory reset then it runs fine. The worst offender on that phone was turning the Wifi on and off. Turning it on (after all updates are installed) causes the phone to freeze for 10-20 seconds.

On the 5.1 the keyboard stops responding to every other key press at times. That’s been my only gripe so far to be honest but it’s incredibly frustrating.