The controversy related to “Apple intentionally slows down older phones” conspiracy is a popular one and many people have agreed that they did notice sluggishness in their iPhones while blaming new software updates.
However, the case is slightly different. Yes, there is some truth to the conspiracy, but the real culprit are not the updates (at least not according to this research) but the deterioration of the non-removable battery of an iPhone is to blame for the performance change.
Some people tried to prove this theory and found out that iPhone’s processor performance declines when its battery capacity is reduced. The issue was first noticed in the iPhone 6 and 6s, people presumed that Apple reduced the performance of the preceding models using their updates in order to boost sales of newer iPhones.
The reducing battery capacity isn’t really Apple’s fault, this is how Lithium batteries work. Their capacity deteriorates upon every recharge. Apparently, the decreasing performance is still a fault at Apple’s end, as the iOS has a ‘feature’ to optimize battery life by examining the battery stats such as its capacity and discharge rate and adjusting the CPU clock speed accordingly.
If it detects a lower capacity, it simply reduces the clock speed to get more battery life and in-turn reduce the performance of the system.
The clock speed in an iPhone 6 or 6s should be 1848MHz. If you have an iPhone, you can check the the current clock speed using benchmark apps such as CPU DasherX. Several people, with older batteries, reported clock speeds much lower than standard – some times as low as 600MHz.
All of this seems to be related to Apple’s iOS 10, which also has another bug that brings the battery level drop from 30% to 1% in a matter of seconds.