SOLVED: Transcoding: When it's triggered and when it's not

[pguidi]

Media B

TranscodeReason=VideoCodecNotSupported, AudioCodecNotSupported, SubtitleCodecNotSupported

Media A

VideoCodecNotSupported, AudioCodecNotSupported

Since your client doesn't support the subtitle format (common on Android TV) it is burning in the subtitles.  Burning in subtitles requires transcoding.  If did not have forced subtitles on B it would probably copy the video stream like with A.

If you need the subtitles, you can use ffmpeg and other tools to extract the subs into a separate file and convert them to a format that your client supports.

Removing all the subtitles streams worked like a charm.
It seems that this issue pops up only for PGS subtitles, with srt it doesn't slow down the playback
I will think up a way to solve this..

Many thanks