How to avoid transcoding?

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Match the media to your client setup. If it's just you, that's pretty easy. Determine what client you're using (i.e., Chromecast, Roku, Jellyfin Media Player on a PC) and what it's compatible with. Most basic setups only have stereo speakers and will play UP TO 5.1 audio. Anything more than that or special codecs like DTS-MA, Atmos, etc... and you're SOL.

Additionally, subtitles don't *always* require transcoding, but anything in the "picture-based" family (DVD, VOB, PGS) generally will require transcoding on most client devices. Some ASS subs will require transcoding as well, depending on how complex they are.

So if you have a real basic setup, aim for H264 Main Profile < Level 5.1 (I think, double-check), AAC audio with 5.1 or fewer channels, make sure it's wrapped in an MP4 container, and avoid anything except SRT subtitles, preferably stored externally as an SRT file (e.g., Awesome Movie (2024).en.srt for English subs in SRT format for media called "Awesome Movie" released in 2024). Happy to elaborate, but this is the main thing folks miss -- your media has to be compatible with all of the clients you serve or your hardware has to be up to spec to handle transcoding.

Somebody will probably come in and tell you an rPi isn't recommended. I agree with that statement, but I say try with whatever you have and learn the basics of media before you invest in upgraded hardware.