Some hardware is barely able to do 4K HDR playback but the defaults in MPV are higher in quality (and thus requiring more GPU) than other players. If you are finding your hardware is dropping frames, you should first decrease the video quality settings in the applications settings.
If you are having difficulty playing HDR content, you should try adding this to your mpv.conf file:
hdr-compute-peak=no
Another possible option that may help is:
tone-mapping=reinhard
There are other options in the MPV manual to consider
