> I use another little-known feature, "ControlMaster". Multiplexes multiple connections into one, it makes making " new" sessions instant

Is this what secureCRT used as well? I remember this being all the rage back when I used windows, and it allowed this spawn new session by reusing the main one.

I'm using that as well but had issues with tunneling where it creates the tunnel in the background and terminates and so you might not know the random port it assigned or I couldn't figure out how to un-tunnel it and tunnel again to the same port. Just bypassed the control master then.

TIL; thanks, that's interesting (and somehow escaped my 20+ years of using ssh)! As usual the gold is in the comments :-)