John Poole writing on the Geekbench Blog about decreased CPU performance as the battery ages:
First, it appears the problem is widespread, and will only get worse as phones (and their batteries) continue to age. See, for example, the difference between the distribution of iPhone 6s scores between 10.2.1 and 11.2.0.
Second, the problem is due, in part, to a change in iOS. The difference between 10.2.0 and 10.2.1 is too abrupt to be just a function of battery condition. I believe (as do others) that Apple introduced a change to limit performance when battery condition decreases past a certain point.
He goes on to note that this is a reaction to a problem iPhone 6s were having where the device would shutdown at 40% or so battery life due to low voltage from the battery.
I agree with John’s assessment that the sudden slowdown isn’t something users would attribute to a bad battery. Instead, they’d feel it’s a processor problem and and upgrade is the only way to solve it. While this may “fix” legitimate technical problem the iPhone is having, now that it’s know to be a purposeful slowdown of the hardware, it doesn’t look good to people who already feel Apple slows down hardware solely to gain more upgrades and make more money.