Performance issue with subtitles

[TheDreadPirate]

They are PGS subs and they are being burned in, which requires transcoding. And the transcoding is being done on the CPU.

Code:

Stream mapping:
  Stream #0:0 (h264) -> setparams:default (graph 0)
  Stream #0:2 (pgssub) -> scale:default (graph 0)
  overlay:default (graph 0) -> Stream #0:0 (libx264)
  Stream #0:1 -> #0:1 (dts (dca) -> aac (libfdk_aac))

There is an experimental setting in jellyfin-web to direct play PGS subs. Settings > Subtitles. Check "Experimental PGS subtitle rendering". From my experience with direct PGS rendering in a browser, the subs will sometimes de-sync briefly.

Your other option is to use Jellyfin Media Player, which has a much better playback engine.