Check the settings in the Plex LG app. Make sure Direct Play is enabled.
The movie needs to be direct playing.
Direct streaming + SRT subtitles results in a video transcode when using the Plex LG app.
Edit:
See post below for additional details. The thread mentions what you are experiencing, but in the context of transcoded audio. It is essentially the same, since when Plex transcodes the audio the video will be direct streamed (see Partially-Transcoded Direct Stream).
