iOS 9 is actually focused on the performance some of old and some of the new devices, which leaves middle aged devices in a pretty bad condition, starting with the devices, (iPhone 5S, iPhone 6 Plus), iPad mini 2, iPad 3, iPad Air, iPad 4. Apple doesn't fix the bugs, I'm one of the users having trouble with the app switcher on my iPad 4 since first beta and I've seen videos of people experiencing the bug most noticeably on iPad 3, iPad mini 2, iPhone 6 Plus, iPad Air 1, Apple can fix the bug and I'm sure about it but it's just about how they think. I've seen also search bar lag on iPhone 5, 5C, iPhone 6 Plus "sometimes", all iPads except Air 2 which is also pretty annoying. All I know is Apple could do everything to provide any feature or fix any bug to the latest iPhone or iPad and some other old devices to show people that iOS 9 is pretty cool but not the other iDevices. Some people still think that the device slows down cause the chip is old which kinda became the biggest lie we tell our selves. I've seen a lot of devices my whole life and all i can say is that all chips in all our Apple devices are ok and some are nearly awesome, it's a matter of coding, maybe it's meant and maybe it's not but the proof it's meant is that I've sent over 100 bug reports and I think only 3% of them were fixed during the entire stage of beta testing. The positive part, iOS 9 has fixed some issues iOS 7 and iOS 8 couldn't, the home screen feels great on nearly all devices and some really bad animations now feel fluid and awesome.