Usually with iPhone/iPod Video upgrades, getting the latest version of iOS with an older device can significantly degrade performance. (And I'm sure it's the same with iPads.)
At least on a Macbook you can reinstall the operating system, but an iPhone 4 is stuck like that.
Us hardware manufacturers are still looking for a fix. Our customers are angry at us and blaming us, and we can't do anything. There's no way for our customers to even tell Apple what's wrong, either.
But you're right: it's definitely not exclusive to iPhones. Planned obsolescence, some would argue.
Is this claim based on anecdotal evidence or some other source?
http://mashable.com/2015/12/31/iphone-4s-ios-9-lawsuit/#l.Zf...
Edit: I'm not sure if this has been fixed with the later iOS 9 versions.
It affects iPad, too.