In reply to Moridin.

first, use your iPhone normally or for example don't use it at all but have it turned on for a whole day or whole battery cycle. then do the same with Low Power Mode. compare the results. you can also compare the Low Power Mode results to your 6 Plus performance when it was still working as it should. if on Low Power Mode it would be at least similar, then you can continue with disabling some features one by one and check which one is draining your battery (as I mentioned in my first comment). if on Lower Power Mode it's still bad, then... well... Apple is doomed. ;)