The performance of iOS9 (9.0.2,9.1) on older devices iPad mini 1,2 is terrible (and to a less degree the iPhone 4s) - no amount of settings tweaks (like Settings-Accessibility->'Reduce Motion' off, Background refresh off, Hard reset etc) as mentioned across the Internet make much difference. There is a substantial lag in performance and speed as compared to iOS8.4.1 or earlier. Don't upgrade if you haven't already!
Note: It's no longer possible to revert to iOS8.4.1 (or 9.0.2 for that matter) as Apple is not signing older versions of iOS prior to iOS 9.1. To explain: Apple cryptographically signs iOS on every single device it is installed onto provide for enhanced security and integrity protection - and secure booting - as detailed in their iOS Security Guide.
Update[May16]: iOS 9.2/3 does improve things somewhat on the older devices (but I'd still advise staying with 8.4.1 unless you need particular iOS9 features/fixes).
