Many of my friends with iPhone 6 and 6S had the same problem.
They were calling me asking if I could replace their battery but I didnt want to risk opening it and buying 3rd party batteries to replace.
And Apple would not replace the battery for free.
They wanted to charge them $80 and were telling them mind as well upgrade to the newest iPhone instead of spending money on fixing the old one that is slow and outdated.
Totally shady since they knew their batteries were crap and instead of replacing all of them for free they were slowing down peoples devices on purpose to get them to buy a new iPhone or pay $80 out of pocket to replace their defective batteries.
Yep we had to pay $80 for a new battery. In so doing they destroyed my wife’s iPhone. Never got that $80 back but did get a new iPhone for free. :/. Very shady on Apples part. Not something you do when you want to keep customers.