What I noticed with my Z3C is that after the second Lollipop update the phone became so hot that I guess the system went in protection mode and the device became quite slow. It had issues on performing regular tasks and the screen was lagging in a very bad way.
A Sony rep can confirm that there is a sorta of protection to prevent overheating of the system. (Spoiler alert: It exsist)
However, none of my apps where updating correctly, no wait.. they were just stuck, not downloading nor installing so what I did? I just stop using the phone, turned off. Wait it to became fresh again and turned it on: All returned working regularly without problems.
Hope this will help people and will avoid unnecessary factory resets.
