I have a different explanation. I've had number of devices slowing down as the time passed, also showing extremely short battery life. With a lot of these devices the problem seems to be inability to fully shut them down. Carring out a power drain method fixed my Tesco hudl 2 and my acer liquid e3. What you do is shut the device down, open it up, disconnect the battery, press and hold the power button for 10-20 seconds, reconnect the battery and turn device on, while doing it I would also do a factory reset. I am aware that taking apart your phone might be a bit much to ask for some users, but with so many tutorials on YouTube I think it's a skill worth having. Good luck and let me know if you had similar experience.
What about iOS on the iPads? I assume that I can't replace the battery in my 10.5" iPad Pro, but I'm in the period when I can still return it...There's nothing in the market right now that is quite like it, and Surface Pro has the Intel-SA-00086 security vulnerabilities that MS hasn't provided a BIOS update for, so I don't see anything else I could replace it with.
More users of older iPhones are now claiming an improvement in performance after a battery replacement. One posted a couple of screenshots on Twitter showing the CPU being underclocked to less than half of its stock speed before returning to normal after a new battery was swapped in.