"In what will come as a surprise to many people, our sources note that even A5-based Apple devices, including the original iPad mini and discontinued iPhone 4S, will be able to run iOS 9. 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."
source
"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."
source