Found my old iPhone 4S running 7.0.4. Safari, Sky Force 2014, etc. ran well with no issue. Only issue was Hangouts just closes when attempting to set up an account so I figured it and possibly other apps require a newer iOS. Also, wanted the improvements like the new keyboard that shows lower case key when caps lock is off. Worried about negative impact iOS 9.2.1 might have on older devices I did a quick Google search but the consensus is that it offers a small improvement so went forth with upgrade. Turned out it was a bad decision. Safari is now intermittently broken and can't render macrumors forum correctly even after resetting and setting up as new. Everything lags such as keyboard and Sky Force 2014 is now unplayable. I'd give up app compatibility and whatever minor improvements to go back to the performance and stability of 7.0.4.