I discovered something odd with mine the other day, I have many different ways of accessing my Plex, chromecasts, Mibox and other random players, and it always seems to buffer, has issues with sound etc that I just can't get to the bottom of.
Installed Kodi on my MiBox, added the plex plugin and everything direct plays. No transcoding.
Plex clients can't seem to manage that.
I like Plex and my server. It's currently my only technical related hobby. I went through a bit of a burnout a few years ago, lost all interest in it. Can't remember the last time I turned on my PC listed in my sig.
Depends on the device, some of the apps have options for the transcoding and what it tries to do their end, others run off the server settings.
I have seen that as well but my main one is that the apple tv app just keeps wanting to play the 720p compressed version even though you set it to 1080p or up and it still streams over the network fine.