I have experienced the same issue, I think it is just iOS11 and not the phone
It's definitely an intermittent problem, as it's performing smoothly again after taking it off the charger this morning. I never noticed this behavior on my 7+, 8, or iPad Air 2 running iOS 11.
I don’t see the issue, or rather I’m unable to replicate it on my X using iOS 11.2 PB4
Watched the video, but wasn’t sure what those actions were trying to accomplish.
I’m just wondering what is happening in the video.It's easier to notice if you're able to play the video at 60fps.