This may just be because mpv has higher-quality default settings for scaling and tonemapping. Try mpv with profile=fast, maybe. To properly compare mpv's and VLC's performance you'd need to fully match all settings across both players.
It was with the fast profile using both software and hardware deciding, important detail I forgot was that the video was av1. Don't have the link to it now but it was from jellyfin's test files
Numerlor|1 month ago