And Apple is getting sued because of it:
https://www.macrumors.com/2017/12/21/apple-lawsuit-slowing-down-old-iphone-models/
As battery performance degrades, Apple limits the CPU performance to reduce "spikes" in power consumption to prevent the device from crashing. Unfortunately Apple does not notify the owner of performance degradation and provide options but it will just force the owner to get a new device instead of getting a battery replacement.
https://www.macrumors.com/2017/12/21/apple-lawsuit-slowing-down-old-iphone-models/
As battery performance degrades, Apple limits the CPU performance to reduce "spikes" in power consumption to prevent the device from crashing. Unfortunately Apple does not notify the owner of performance degradation and provide options but it will just force the owner to get a new device instead of getting a battery replacement.