> If you pick embedded subtitles with these clients, Jellyfin must extract the subtitles files immediately, and this takes a while depending on container format, file sizes and server load.

Really? How?

My expectation would be that extracting a subtitle file takes a few seconds at most. If you had to do it on the fly every time you watched anything in your library, it wouldn't matter.

I don't undestand how movie containers work, but I ran this on a 70 GB movie file to test:

    ffmpeg -i input.mkv -map 0:s:0 /tmp/subtitles.srt