One of the reasons is that it slows down is because the battery is actually not able to deliver enough power to run at full speed. My iPhone 6S would randomly shut down when I was using significant power (I.e a CPU hungry app) even at 70+%. When I got to less then 40% almost any app could cause a sudden restart. Apple apparently fixed this problem in a software update by limiting the power the devices draws, which fixes those random restarts for the most part but slows it down significantly.

Fortunately, I could replace my device because that was the case (I went to Apple and they replaced my battery after they did some extended tests because they were aware that a “small number of iPhone suffered from this issue and they tried to find out which ones“). Sure enough, about one or two months later I checked my old serial number and apples website told my that my old phone was eligible to be replaced.

This seems to affect the iPhone 6 and 6s the most. According to geek benchmark my used iPhone 7 is at 80% speed when it has about 50% battery remaining and at pretty much full speed when my battery is fully charged.