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I've been having this problem on and off for a long time. Sometimes when the YouTube app's video quality switches to 60fps HD, the framerate actually drops to like 2fps instead of going to 60fps. I'm really quite sick of it and it prevents me from watching hd quality on videos with high framerates. When I go back to 480p it goes back to 30fps like it normally is.
I always update my apps and my ZTE software updates.