Comment # 13 on bug 1129035 from Stefan Dirsch

Ok. Last thing you could try is uninstalling intel-media-driver package and
install intel-vaapi-driver. In case things work better with old libva backend.

Otherwise I don't know either. We could try to reproduce the issue here with a
Dell 7280, which unfortunately still uses Skylake instead of Kabylake. :-(

Possibly things are not related to gfx drivers at all. GTM may work better with
Firefox or Google chrome (the proprietary one).