As suggested by Reddit users, Apple really reduces the performance of an iPhone with a worn out battery. According to the company's representatives, the limitation of the processor's clock speed is the insurance against the sudden shutdown of the smartphone.
How it works?
The slowdown is explained by the peculiarities of lithium-ion batteries. At aging, low level of a charge or on a cold the element of a food does not maintain high loads. At the peak of performance, when the processor consumes most of the energy, the battery simply can not provide the required current, and to protect the internal components from damage, the iPhone can turn off.
To solve the problem, last year, Apple added a feature on the iPhone 6, iPhone 6s and iPhone SE, which smooth out the sudden peaks of chipset consumption, when necessary. The system appeared on the iPhone 7 with the iOS 11.2 update, and the company is going to use it in other devices.
The Apple statement explains why in the above discussion thread on Reddit , iPhone 5s owners did not notice a significant drop in performance over time (at least in benchmarks).