Is the audio transcoding?
With the Plex LG app, if the audio is transcoding, enabling any form of subtitle results in a video transcode.
Also, is the TV HD, not UHD? (specs at flatpanelshd) If so, it is not officially supported by Plex. From the LG section of Which Smart TV models are supported?, “No 720p or 1080p (“full HD”) sets are supported for our current app at this time.”
You might consider getting a Amazon FireStick, Roku, AppleTV, Nvidia Shield, etc. Since version 8.0.0, Android TV devices direct play subtitles even if the video or audio is transcoding. I’m not sure how Roku or AppleTV handle subtitles, but probably much better than the Plex SmartTV apps, which are limited at best.
