I wish #matrix the best, but #XMPP #Jabber still rocks:
> If deployed on a 16 GB RAM machine with at least 4 cores, a single ejabberd node can typically handle 200-300 K online users.
From: https://docs.ejabberd.im/get-started/
My 17 years old server is on 8GiB RAM with four 1.2GHz cores. I don't notice it. User data takes up 200MiB of space (it massively increased recently for some reason). Unlike Synapse :)