I managed a big fleet of iOS devices (hundreds) around the iOS 6 to iOS 10 time period when battery drain was a common complaint (https://news.ycombinator.com/item?id=13054056). Completely wiping the phone and setting it back up without restoring the backup fixed those battery drain issues 100% of the time. If you wiped the phone and restored the iCloud backup the battery drain would persist because you restored whatever glitched config file was causing it.
