This is indeed the correct answer. It is under the transcoding options. Your server will essentially need to re-transcode the video on the fly to add in subtitles. Which requires a not insignificant amount of processing power.
I ran into this issue while running jellyfin on a raspberry pi.
So long as your server has hardware acceleration turned on. To allow jellyfin to use your GPU for transcoding. You should be fine.
jellyfin with subtitles on Chromecast angielski
Hey fellow saltdogs! :D...
Baldur's Gate 3 Patch 2 will be "chonky," with fixes for performance issues and other issues (www.gamescensor.com) angielski