But the upgrade doesn't always make that happen. It's only if the battery is in poor condition, so that it can't retain enough charge to handle a sudden surge in demand from the processor. (Phone processors don't work flat-out all the time; it helps the battery life enormously if they cut their power demands until it's needed.) The deliberate slowdown is being treated by consumers as evidence of a grand conspiracy by Apple to make people upgrade their phones. But consumers are wrong.
What used to happen was that you'd be using your aging iPhone, you'd upgrade the software, and then when you were getting low on charge, and, say, you began playing a game that made big demands of the processor, then the phone would abruptly shut off.
When Apple realized this was happening (because it could see it from the data sent anonymously back about phone performance), it added some code
into later releases, so that if the processor made those hefty demands of the battery, it would be slowed down. That seems to have been added in February 2017, with iOS 10.2.1, which stopped "sudden shutdowns" of iPhone 6S models.
Yes, Apple is doing it on purpose. And it hasn't been telling people about it; people have been inferring that something is slowing their phones down, and that it's the new software. Correct, so far.
But it's the third step -- that Apple is doing it to make you buy a new phone -- that's wrong. Apple's actually trying to help you hang on to your old phone for as long as possible, but it has screwed up how it tells you about that. In fact, it hasn't told you that at all, which is poor communication.
Apple has had years to explain this, but hasn't. There isn't even an onscreen message or display until your phone's battery is perilously close to permanent death.
The crazy thing is, if you're using an Apple laptop, then you do get a message telling you that the battery is dying. That's what happened a few months ago with my 2012 laptop. I took it in, got the battery replaced, and pretty much had a brand new laptop, cleaned and everything.
Clearly, Apple does know how to tell its users the battery on their fabulously expensive device is dying, or even ill. But on the phone and tablet it doesn't. It should. Apple says its phones are "designed to retain up to 80% of their original charge capacity over 500 complete charge cycles."
For the iPad and Apple Watch, it's 1,000 charge cycles. So, it's probably not surprising that people complain about their phones seeming to slow down after 18 months or so, and definitely after two years: By then, you're well past 500 daily charging cycles. (For the iPad, it's about three years, which is probably why you don't hear those owners complaining in the same way.)
So, iPhone owners generally won't see a slowdown the first time they get one of Apple's big annual updates (eg an iPhone 6S owner going from iOS 9 to iOS 10 during 2016) but will the next year (to iOS 11).
Apple does offer a battery replacement program, which costs $79 in the United States. I've seen some people on social media suggesting it should be free on phones over a certain age. I suspect Apple CEO Tim Cook, and Wall Street, would faint at the idea: The more phones Apple sold, the bigger the liability would be; and no rival offers free batteries. Apple might be exceptional, but shareholders would view this as simply setting fire to money. (My laptop's replacement wasn't free.) As ideas go, that's definitely one that deserves an early shutdown.
One question that's raised here: Why don't owners of Android phone complain about slowdowns? Where's the social media outrage about that? Surprising fact: They do. If you search for "Android update slower" or similar, you'll find people grumbling on Twitter and elsewhere about how a new update has slowed down their phone. But because Android phones don't get the monolithic upgrades that Apple does -- in fact, it's almost pot luck if such phones ever get upgraded (with the extra security and new features such upgrades bring) -- you don't see the same noise about it.
The fact is still that Apple has done a poor job of explaining to people what it was doing, and why. Figuring out how to improve that will be something for Cook and the team to chew over along with the holiday meal. As lots of technology companies have learned over the past year, it's not enough to write clever code; you have to find the best way to explain it to the humans who will be affected by it.
But at least now you have an idea why your email is so slow to load. Then again, was there anything in there you really wanted?