You touched a very import point "What are they running in the background that is so power hungry?" , iOS10 killed thousands or millions of batteries world wide, they aknowledged only the battery problem by the 6s and only for the ones manufactured until october 😟
It was not only the iOS itself, but the several Apps we have intalled in combination with the iOS did the effect ...
the reset of the network setting is important, it worked for me very well, I think it was related with my phone provider, the software used by them probably were not correctly updated during the iOS installation.
the next very important was the Apps from Facebook and respectivily their Messages. although they were setted to not running in the background, and also they were not running at all, they are consuming you iPhone resources. killing those 2 Apps gives a new life to your iPhone.
probably there are more Apps which should be killed, I did not found other so obvious as the Facebook and Messages.
LESSONS LEARNED:
when we buy such equipments, never upgrade the iOS to the next level. why not:
- Apple will never allow us to go back even if we want, then we will be stucked in the last version
- The equipment (iPhone, iPad, AWatch) we bought were developt together with their own iOS, the next iOS is developt for the new hardware, to push the new hardware to its limits, and for the old hardware will be adapt to just continuing running.
- never forget, these companies, they develop the equipment to be running for 2 years, then they are consider as replacable to the next generation.