WRT PlexWeb:
Chrome does not support H.265, so playing a H.265 video forces a video transcode.
Chrome has limited audio support, so your audio may be transcoding as well.
Chrome does not support image based subtitles - PGS/VOBSUB. Selecting such will require PMS to burn in the subtitles, which forces a video transcode.
Not sure about FireFox. I do not have it on my system. Edge has other issues which cause problems with Plex and is best avoided.
If playing on a Windows or Mac, use Plex Media Player. It is a much more robust application and does not have the limitations of web browsers. More of your media will direct play and you will see less buffering, etc.
Do not use the Plex app from the Windows Store. It has not been updated since May 2017.
