I saw it happen while playing a game and the lights were turned off and the screen started dimming. It happens on my iPad Mini 4 a lot because that's what I play games on.
I've heard others talk about lag during screen dimming and I kinda was like "Yeah right" until I updated my iPad mini 4. Everything was fine at first and I wasn't even thinking about the auto-dimming lag, but then the right conditions hit and I was playing a game while my iPad was dimming and sure enough, I got about 10-15 seconds of bad stutter in a game that is otherwise flawless 60fps.