Like some people mentioned here, the irony is that 32-bit devices are actually performing better on iOS 9 than 64-bit devices. I was trying out the 6s at the Apple Store yesterday and I saw lag pretty much everywhere. Control Center, App Switcher, Spotlight Search, etc. It's not the 6s itself, because it's ridiculously fast. It has to do with issues with Metal on iOS 9. It's the only hypothesis that makes sense, because iOS 9 runs really well on the iPhone 5/5c (the 4s is already really slow, so that's the only exception) and the iPads below the iPad Air with the A7 that supports Metal run it well as well.