I have the same machine and can state that's not normal.
iGPU in this Macs are fine to handle a single monitor running 4K @ 60Hz. One can say it's enough to push 5K monitor too.
The first thing that comes in mind looking your videos is the scaled resolution. Are you running scaled resolutions on any display?
Are you using them both on?
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I'm running the 10.13.4 Beta and this beta has addressed several performance and glitch graphics related to Intel iGPUs. If you are experiencing such thing, you should try it.
The Public Beta is pretty stable. You can tray it. Don't forget to Time Machine your Mac first!