Subtitles force transcoding a lot of the time depending on playback situation and media. A lot of compatibility issues there - downloaded movies often will have the PGS subs from a bluray, anime can often have ass/ssa, ibx subs, etc plus they all tend to be embedded in the media (and in anime’s case often with extra fonts and attachments).

all of these (basically anything but external srt) can interrupt direct playback depending on users Jellyfin client. Some have better sub support than others, android vs apple vs some esoteric client like webos all have their own niche weirdness. You need to search what the best option is for each users scenario. Like for my users that have apple devices checking this box in settings>playback allows direct streaming of a lot of content:

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But for “the problem show” what do logs say? How is it encoded? Quick sync can’t transcode everything. The older your processor the less it can do and niche formats it definitely can’t do at all. Like unless you have a real new cpu (13th or 14th gen) it’s not doing av1 or vp9 content and that’s still getting offloaded to cpu for transcoding. And if it’s some ogg vorbis thing or whatever it won’t work. And do logs verify igpu is being utilized for transcoding/mapped correctly? It can be a bit of a pain in the ass (unless they improved the process, it’s been a while since I’ve had to do it)