Dunno about Plex, but Jellyfin supports AV1 direct play just fine. Just not on Roku, which I mainly use it on. I’m a small-time contributor on the Roku version, though, and there’s progress towards AV1 support, at least.

On ffmpeg, yeah. I can get close to real-time encoding with the new version of libsvtav1 and I save space with around the same visual fidelity as x265, at least, in my experience. If you want to try it out, I recommend using the ab-av1 tool, which automatically finds the best CRF to VMAF for encoding.

edit: Transcoding speeds, I don’t find that it’s slow, even if I’m using software for transcoding, though I’ve only been using it for my Jellyfin server for about a month or so.

It really is awesome. Lots of leaps forward for AV1 recently. It encodes faster than x265 in some situations with so much space saved. It’s still in the early stages, really, and the compression isn’t perfect, but for video streaming purposes, I’ll take it over x265 any day.

I have a personal Jellyfin server, and I usually reencode from x264 to AV1. Though if it’s a matter of choosing a source, I always go for x264 for the least compression.