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It's nice that Jellyfin supports ASS subtitles without transcoding, but I have another issue with it: it takes about a minute for the subtitles to actually show up after starting playback. Don't know if it's an issue with SubtitlesOctopus (which should work fine...) or Jellyfin itself
Note: This is only on the Web Player. The Desktop Media Player uses mpv, which loads the subtitles instantly.
I figured it out: the issue is with extracting the .ass subtitles from the video file.
A video player like VLC or MPV only has to load part of the .ass subtitles. On the other hand, Jellyfin needs to extract the subtitles from the video file in order to provide it to SubtitlesOctopus. This is what actually takes a lot of time.
Update to this issue: I just realized that Jellyfin has a "Subtitle Extract" plugin that does exactly what I want to do: extract SRT and ASS subtitles (and convert other subtitles to SRT).