I use a Skylake on a 4K laptop with a 4K external monitor above it. It is plenty fast, with no tearing. Under Qubes, where app VMs don't get to use the GPU, mpv plays full-screen 1080p video using 3 cores.

(Oddly, the external monitor will do 4K only at 30 Hz. I don't know if that is a limitation of the laptop or of the monitor.)

The 30hz thing might be because of an older hdmi version, like 1.4. I have had the same issues and it was a hardware limitation