Get the battery replaced on your 6S before Dec 31 for $30 and it will perform like new again. Mine was at 87% but it made a noticeable difference in everday usage.
Just did this on an SE, I was expecting to come back and pick it up in a couple of days (website says up to five business days when you make an appointment), but it took a bit over an hour instead. Having an Apple store is great.
