MacBook Air display starts lagging after some time while plugged into LG 4K display

Hi all, I just found an interesting issue with my new Air. When it is connected to an external monitor (LG4k) time after time I notice that the built-in screen starts lagging (Not external)

It is not connected with the performance, CPU, and memory aren't busy, no high-performance peaks or so. It's just lagging. The cursor moves well I even can drag the window to the connected monitor and it works with no lags at all.

Have any ideas? Check out the video

[Re-Titled by Moderator]