Apple got into a lot of heated debates with tech reviewers and the general public as the company got caught out deliberately slowing down the performance of the CPU of older iPhones without the consent of the user and owner.
But this is why Apple decided to do so:
Apple uses Lithium-Ion batteries in their mobile devices which have a reputation of losing their efficiency and maximum charging capacity over time this doesn't necessarily mean that a user will only experience battery issues, but performance issues as well. So what Apple decided to do is throttle the maximum CPU performance of older iPhone models in order to favour battery consumption.
The problem that the public had with this is that Apple didn't inform their customers that this was the approach they were taking to try to resolve this issue, and they just went ahead silently and did it. Furthermore, many users feel that they should have been given a choice over whether they favour battery consumption over performance and vice versa.
Take a look at the video below by the YouTube channel: Apple Explained on Why Apple Slows Down Older iPhones...