Alternately, consider using an Nvidia Shield Pro instead of the Plex app on the LG. An Amazon FireStick 4K Max is also an option if you do not need TrueHD/TrueHD + Atmos audio.
The Plex app on Android TV devices is a more capable client. It will direct play PGS, VOBSUB, & SRT subtitles whether or not the video or audio is transcoding. ASS subtitles will cause a video transcode, but that is the case on most Plex clients, including LG.
See the post linked below for additional details.
A bit of context: It mentions “burning” subtitles, which is the process of adding them to the video stream. The process uses the CPU, not the GPU. Any low power CPU, such as a Celeron, will struggle to burn subtitles in real time. Also, a Plex Pass will be of limited help. The video transcode will use the Celeron’s integrated GPU, but the burning still uses the CPU.
