I have been experiencing similar issues as well documented here:
Recently I have just been using Jellyfin to play any high bitrate files and plex for everything else. I really wish Plex would offer additional player support such as external players or libVLC like Jellyfin does as I find their interface lightyears ahead of Jellyfin or Emby, but constantly dealing with buffering for high quality files is a big tradeoff.
Has anyone found any tips or tricks besides just not using Plex?
