Just because something sounds like a conspiracy theory, it doesnt mean its 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.
Heres the evidence presented by Reddit user TeckFire and his iPhone 6s:
iPhone 6s with the old battery and the new 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.
Theres 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 chipsets clock speed, it should be 1848MHz. Many redditors report speeds as low as 600MHz.