It appears that some iPhone 6 or iPhone 6s family devices may see slower processing speeds as a result of a battery that has reached the end of its operational life, with some users getting a speed-up upon replacement -- but the slow-down and consequent speed-up isn't universal, and there may be other factors at work.
A lengthy Reddit thread was started on Dec. 10, with several satellite threads spun off over the weekend. All of the threads had users claiming higher benchmark results after a battery replacement. While there is no universal improvement in benchmarks after a replacement, some additional users did confirm that their devices felt faster after a replacement.
Confusing the issue somewhat, some users with replacements at a Genius Bar saw no improvements. Additionally, users with third-party batteries sourced from unknown vendors also saw no improvements.
At present, the theory is that the iOS 10.2.1 update issued in part to rectify iPhone 6 shutdown issues with a low-power battery condition implemented some kind of down-clocking routine to slow the processor in afflicted devices. The thread speculates that the same routines to prevent the premature shutdown exist in iOS 11, and are what is causing the less-than-expected results in benchmarks.
Users theorize that Apple addressed random shutdown issues by down-clocking the processor in afflicted devices.
The down-clocking of the processor in the iPhone 6 and iPhone 6s family devices reportedly extends to even when the iPhone is plugged in. If universally accurate, this is not dissimilar to how some iBook models after the Intel migration would run slower when running on the AC adapter without an installed and at least partially functional battery.
AppleInsider attempted to get clarification on the situation from sources within Apple. Apple service guidance information provided to us by sources not authorized to speak on behalf of the company do not have a replace battery step as a rectification step for user-reported slow iPhones.
We were unable to get any information explicitly confirming or denying the theories postulated in the Reddit thread. We were told that customers should get their batteries replaced at Apple-authorized service centers, as that is the only way that it can be guaranteed that a battery meeting all of the company's specifications can be met.
It is not clear if other families of iPhones are impacted by the same effect.
As a side-effect of the thread, and consequent reporting of it, the conspiracy theory suggesting that Apple intentionally slows down older iPhones to force purchasing a new device has risen again. While it has been conclusively proven that older iPhone hardware with an adequately functioning battery is no slower than it was at launch, any routine to down-clock an iPhone processor in an environment where the battery is weak can be dealt with by a battery replacement -- without mandating a new iPhone purchase.
A battery replacement through Apple costs $79. A new iPhone SE on the low-end costs $349, with the iPhone X on the high end retailing for $999 and up.
Should AppleInsider get any more information from our sources within Apple regarding the possibility of a processor throttle with a chemically depleted battery, we will update accordingly.