We have a number of iPads in the family and given everything we now know about iPhone batteries and performance it was reasonable to ask if Apple is doing the same to its tablets. The short answer is don't worry, Apple isn't throttling your iPads performance based on the age of its battery. And now for the longer response. Power Management is a complex balancing act which is further compounded by an aging battery that is less able to provide power quickly enough to the system that needs it. This was the cause of unexpected iPhone shutdowns that Apple addressed a year ago in the its iOS...
Source: https://www.gsmarena.com/are_apple_slowing_down_ipads_with_older_batteries-news-29098.php
