On my iPhone 6, I've noticed a few things about iOS 9.1 beta 4.
1.) very stuttery control center animation.
2.) very stuttery multitasking switcher.
3.) unlocking phone has lag occasionally.
4.) quick replies not only have a SUPER laggy animation, but often it will hang for at least 10 seconds not allowing me to answer.
When I say lag or stutter, I recognize that the animations are NOWHERE near 60 fps. I have never had this much stutter prior to iOS 9. If I had the choice, I would have gone back to iOS 8 but it's too late 🙁