Back in December of last year Apple has confirmed that the company is doing some optimizations to iPhone smartphones with batteries that are close to the end of their life cycles, which significantly reduces the performance of the phones.
The official explication is that an old battery might not keep up with the energy requirements of the device, causing the phone to shutdown at random times when the phone is used for more demanding apps. The solution found by Apple was to artificially limit the energy usage, and this also has an effect on the performance of the phone. The news that Apple intentionally brings down the performance of older phones has caused a lot of controversy, and some even blamed Apple of not having the best intentions.
The solution Apple came up with was to reduce the price for battery replacements and promising to create a feature that will allow users to disable the optimization that reduces performance in a future iOS version.
What is certain is that the drop of performance of affected devices is more noticeable in the day to day use, which was also confirmed by many performance tests after the battery was replaced. Basically, from a 2485 score in GeekBench multicore, an iPhone 6S can reach a score of 4412 points, which is almost double.