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. Here's the evidence presented by Reddit user TeckFire and his iPhone 6s: iPhone 6s with the old battery and the new...
Source: https://www.gsmarena.com/an_old_battery_may_be_slowing_down_your_iphone-news-28665.php
