First you need to find out exactly what the plex server is doing when your video is buffering, you can find this @ Plex Web > settings > status dashboard
this will indicate if the video or audio is transcoding and whether subtitles are enabled.
As already posted previously, current smart tvs do not generally support the HD audio, so you must use a dolby digital or dts 5.1 or stereo audio track.
Some subtitles also can cause transcoding, so make sure NONE is selected for subtitles.
Finally, please read the entire thread @ Plex, 4k, transcoding, and you
