We'll see if we get a video with iOS 8. But based on performance in other areas under iOS 9 I wouldn't hold my breath it's the apps fault.
Protube has also an Pip Mode and works just fine on the 5s with ios 9.1. And on the mini2 metal games are also performing reasonably well. So i doubt this particular stutter is apples fault. Blame them for the rest, but please blame also Google for not having an optimized app (still no official pip on the mini2).