In order to avoid the sluggishness and bugginess that was most notably seen in iOS 7 for the iPhone 4, Apple has restructured 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. Thanks to this new approach, an entire generation (or two) of iPhones, iPads, and iPod touches will be iOS 9-compatible rather than reaching the end of the iOS line.
Great news. In addition to better performance on older devices:
- slow iPhone/iPad upgraders will be able to run the latest versions of third-party apps
- older iPhone/iPad users will get the latest security updates
- app developers can spend less time/resources on supporting older iOS versions
- third-party apps will progress faster