Last year many iPhone 6S users reported problems that resulted in the iPhone 6S shutting down at random times. After an investigation, Apple said that the culprit is a lot of flawed batteries that ended up being implemented in some iPhone 6S smartphones and Apple offered to replace the batteries for free for the affected phones.
But the final remedy came a few months after after Apple admitted that more users were affected than anticipated. The new fix was a software one, included in the 10.2.1 update. According to Apple, this has reduced the problems by 80% for the iPhone 6S and by 70% for the iPhone 6.
At that time the official explication was that the software fix only adjusts the power requirements thus protecting the battery. But based on investigations made by users it seems that iPhone 6 Plus devices updated to iOS 10.2.1 lost a lot of processing power. From a score of 1471 / 2476 points it dropped to 839 / 1377 in the Geekbenck benchmark test.
By artificially slowing down the devices Apple has resolved the battery problems but this has been achieved at consumers expense as they now have devices that do not perform as they should.
We have to mention the fact that not all iPhone 6 / iPhone 6S currently in use have this problem, only those known to be equipped with flawed batteries and the only real fix to this problem is to have the battery replaced.
Apple says that the battery of the iPhone is designed for a lifespan of two years and many of the iPhone 6 and iPhone 6S units currently in use have passed the 2 years time frame.