The main limitation is not the server's processing power; it's running on a VPS with 6 dedicated cores from an AMD EPYC 7702. The reason it can take a bit is that there's actually a lot more than transcoding going on in the background. While recording, all the video feeds are stored along with chat logs, the presentation itself, and timestamps for the slide changes. After ending the recording, all of that is mixed together so, when replaying the meeting, you can skip around by clicking the presentation slides to the left and see everything that was said in chat as well as the presenter's video feed to the right.
I wrote about a good portion of this in the docs
I haven't used it so can't say :/
I prefer the UI of BBB
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