A while back, users of older Apple iPhones started making noises about their phones being "throttled" and running slower.
Turns out they were right. Apple had found that, for certain applications, if the batteries were older (and possibly dying) the demands of the application could cause the phone to simply quit, and stop working. So an upgrade to the operating system checked for these conditions, and, if the battery showed signs of failing, would dial back the CPU cycles so that the crash wouldn't happen.
Trouble is, they didn't tell people first, didn't allow any options, and people got upset.