Just because something sounds like a conspiracy theory, it doesn’t mean it’s not true. Many people complain that their iPhones get slower and slower with every iOS update, but it turns out that they may have been blaming the wrong thing.
It may be the battery – as it ages and loses capacity, the iPhone reduces the clock speed of the chipset to maintain battery life (which obviously tanks performance). And the cure is obvious enough, just change the battery.
This seems to be related to the iOS 10.2.1 update and the notification “Your iPhone battery may need to be serviced”, which itself is related to the 30% bug – an issue that caused the reported battery charge to drop from 30% to 1% in seconds.
There’s evidence that the iPhone 6 and 6s generations slow down but the 5s and 7 are unaffected. You can use the CPU DasherX app to check the chipset’s clock speed, it should be 1848MHz. Many redditors report speeds as low as 600MHz.