It would be quite interesting to see if someone can do a running comparison at increasing battery wear levels - 100, 250, 500 cycles and so on. (macrumors.com)
It basically charges phone after learning your charging habits to supposedly prevent battery wear off. (suggestphone.com)
However, he also suggests that Apple's change in iOS 10.2.1 to address unexpected iPhone shutdowns has also artificially limited performance as battery wear passes a certain threshold. (slashgear.com)