When I first read your request, it struck me as wistful thinking, especially after you got down to the part about running on really old hardware. But then I remembered.... VLC. VLC has been around since ~2000, and it has every possible combination of open-source video/audio codec, and you can open capture devices, and stream to the network, and choose whatever bit rate would allow that to work on a limited connection, and it is written in high-performance C (especially the older codecs) and could run on any computer in the last 20 years.
It would make for a clunky video conference system, but you could have each user open VLC to stream their own camera, and then open one VLC player for each user you wanted to see video from. So for 2 people, 2 copies of vlc, one outgoing and one incoming. Beware of feedback loops.
If that works, you could try scripting it into a more cohesive experience. As it happens, I'm the author of VideoLAN::LibVLC which would allow you to start multiple copies of libvlc from within a perl script. I designed that library for being able to dissect the individual video frames though, which won't be useful to you, but there might be enough API available to make a video conference app. There is also Vlc::Engine and VideoLan::Client.
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