Since 9.0, I've noticed that just about every animation has to be run at least once before it will animate smoothly. After a reboot, the first unlock stutters, then the first app opening, the first time you open safari tabs... the first time you do ANYTHING in iOS, it will stutter, but then it will be fine until you reboot again. Why is this? I've never seen this behavior before in any other software before iOS 9. It's like these new animations need to be "primed" or "cached" before they can be ready to play nicely.