We feel compelled to point out that devices on the borderline - the somewhat older iPhones and iPads that only just make it on the list - have in the past often struggled to run new software effectively. The iPhone 4s was the oldest iPhone that was allowed to run iOS 8, but lots of users found that it slowed down as soon as the installed the new software. iPhone 4 owners had a similarly negative experience when they upgraded to iOS 7.
These devices were powerful enough to run the new version of iOS - but only just. And you don't want to be in that position. We therefore used to advise people not to update their iPhones and iPads if they were at the older end of the list of compatible devices. It's generally very hard to downgrade from one version of iOS back to the previous one.
That said, normally iOS gets more and more demanding to run each year; but Apple claims iOS 9 isn't just level with iOS 8, it's actually less demanding. In other words, devices that were starting to run a little slowly with iOS 8 may even speed up when upgraded to iOS 9.
How to update to iOS 9: Should I upgrade to iOS 9? What are the benefits, and will iOS 9 slow down my Apple device?
This leads us to a harder question: even if you're allowed to upgrade, is it a good idea to install iOS 9? Are there any reasons why you wouldn't upgrade to iOS 9?
Should you update to iOS 9? The cons
First of all, bear in mind that upgrading iOS tends to be essentially a one-way journey. It's always extremely hard (if not impossible) to go back to the previous versionafterwards, so be sure you want to do this before starting the upgrade process.
So you'll probably be stuck with the new OS if you update. But are there any actual down sides in the way iOS 9 will work?
The first is speed. While Apple always works on streamlining iOS and making sure it runs smoothly, it has to be said that the past couple of updates have both slowed down older iPhones and iPads. As we said, iPhone 4s owners were not impressed when iOS 8 noticeably slowed down a lot of their devices, so the same could be true for iOS 9. Apple says this year will be different, but we'd still advise caution until you've heard from people on the same hardware as you that iOS 9 runs okay.
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