Yes, mpv/smplayer should be made the default media player in Qubes OS’s default templates, because Gnome/xfce players are slow and bad. I was surprised how out-of-box video-players are bad in Qubes OS, especially considering it uses only CPU to play video.
I even posted something on github or user group to offer such change. Unfortunately it did not happen, maybe because it’s Qt software and Qubes OS currently stick mostly with GTK stuff.