Can’t comment on the Samsung app. I don’t have a Samsung and do not know their specifics.
With the Plex LG app, if the media is direct streaming, such as when audio is transcoding, enabling any subtitle type results in the video transcoding and subtitle burning (added to the video stream).
Plex does this to keep the subtitles in sync with the video and audio streams. The root cause is a limitation of the streaming protocol used between the Plex server and the TV.
The workaround is to choose an audio track that does not transcode if one is available.
The alternative is to use a Plex client that does not have the limitation, such as Android TV or AppleTV. Roku devices support SRT but not PGS subs. Enabling PGS subs on a Roku device forces a video transcode.
Is your server capable of hardware accelerated transcoding the video? If so, it should use the GPU instead of the CPU to transcode the video.