NetBSD on Intel HD Graphics has always had slightly choppy/slow video playback with screen tearing by default for me. Tested on Intel HD Graphics 530 and 4400. The way I always fix it is changing the default Xorg settings (accelmethod from sna to uxa) and force enable acceleration in Firefox by going to
about:config and toggle the
layers.acceleration.force-enabled to true - video playback and scroll in Firefox etc gets butter smooth. Maybe this could help improve your situation a little bit?
I don't remember exactly which Xorg option it was, but on my previous computer changing settings like "SwapbuffersWait", "TripleBuffer", "Vsync" etc to false also improved my situation.
You can generate a default Xorg file and play with the settings by running
It's also worth noting that trying a current kernel might also help.