I think it's just a rumor. If Apple fixes things and speeds up UI transitions to 60 fps on all devices, there will be lesser incentive for many to upgrade to newer model of iPhone. My prediction is that all current lags and stutters will remain unchanged in iOS 9 but Apple will make double, triple sure that it does not show performance problems on iPhone 6S. I also expect all A6 devices artifically brought to a crawl with iOS 9. That's it. Ultimately it's money above anything else.