My household has both an iPhone 6 and 6s. Both have become noticeably slower, more battery consuming.
I just did the full backup and restore trick. We think it worked.
https://daringfireball.net/linked/2017/12/22/glenn-iphone-sl...
http://mglenn.com/blog/2017/12/22/apples-bungled-battery-fea...
My boss (she who must be obeyed) and I both "feel" that our phones are now more responsive and some features have resumed working like normal.
Such as retrieving voicemails, which had become unresponsive. For both of us. I had been blaming our cell provider (carrier). My bad.