I'm sure this must have something to do with the LG TV Plex client app and its compatibilities with subtitles and codecs as I am not seeing any issues on other platforms such as iPadOS, Windows, iOS and Android. HOWEVER, this does still not explain why: if I have NO subtitles on, it directly plays without buffering, regardless of video and audio codec. The Handbrake 'solution' is far too inefficient especially with lots of media files. So, I've used Handbrake to change the audio codec from OPUS to AAC as well as the video codec from HEVC to H.264, and I'm able to direct play the file with SRT subtitles. UPDATE: After a lot of digging through reddit and Plex forums about similiar situations to mine, I have realised that the audio codec could be the reason for transcoding. Why is that? What can I do to fix this issue (apart from getting a more powerful server)? However, when watching the same content on my PC (fast) and on my iPad Air 2 with subtitles, it doesn't buffer, at all. If something doesnt play then plex should usually transcode it, is your TV connected via ethernet to the Plex server. I use a NAS (WD M圜loudEX2 Ultra) as my Plex media server and when watching with subtitles on my LG Smart TV through the Plex app, the video buffers quite a lot and I'm aware that this is because of transcoding and my NAS doesn't have the processing power to transcode those files (the log files state the speed is 0.3 which is < 1.0 and the minimum speed it should be at).

Plex buffering lg tv