For the Plex app on LG WebOS TVs, enabling PGS or VOBSUB subititles results in video transcoding. SRT subititles will direct play. However, if the audio is transcoding, enabling subititles results in video transcoding.
Basically, avoid TrueHD audio, as it is not supported by the TV. AAC/AC3/EAC3 (+ Atmos)/dts/dts-HD MA audio will direct play. When playing dts-HD MA 5.1/7.1, the Plex app only uses the dts 5.1 core audio. The -HD MA part is discarded.
On Android TV devices, transcoded audio + subtitles results in transcoded video (see post below).
