Unreal: Slow partial changing of picture while rotate neck.

I am using Unreal Engine 4.9.2.
Why the picture on the rift changing so slowly so I can see the black borders of previous image.
It’s not the case in demo test, but in Unreal it exists.
There is no such effect in the mirror texture on the screen.

I also noticed black vertical parts of image, like in the old cinema, but wider.
Totally like it. In demo I did not saw that.