Apple's iOS updates have a reputation for degrading the performance of slower devices. On one hand, it makes sense — more features require extra grunt. On the other, given the strict range of phones and tablets the company has, surely it can take a bit of time to tune its updates for specific hardware? Well, iOS 9 could signal a shift in Apple's attitude towards optimising the platform for the likes of the iPhone 4S.
Citing "sources within Apple's software development departments", 9to5Mac's Mark Gurman reports the company is spending a lot more time optimising iOS 9 to run well on ancient (relatively-speaking) devices. The hardware in question appears to be that running the A5 processor, so if you have an iPhone 4S, iPad 2 or mini, you could be in for a smoother experience come the next major iOS update.
Gurman writes that this is a result of Apple "[restructuring] its software engineering process to better support older hardware":
Instead of developing a feature-complete version of iOS 9 for older hardware and then removing a handful of features that do not perform well during testing, Apple is now building a core version of iOS 9 that runs efficiently on older A5 devices, then enabling each properly performing feature one-by-one.
It seems like it's in Apple's best interests to keep users on the hardware treadmill, but perhaps it doesn't want to get too ahead of itself in the feature department. That, or it doesn't make much sense to say you technically "support" a device with a new iOS version, yet the thing runs like an upside-down turtle once you update.
Whatever the reason, I'm sure everyone with an iPhone 4S is looking forward to putting off that handset purchase for a generation or two.