Matrix is too dependent on it's only vendor in existence, their only server in existence has performance issues. Also, a monolith standard is hardly viable for federated networks, where all nodes can't upgrade all at once.
Ah, I did not realize you were referring to the software. In that case, there is also Construct[^1]. There are other implementations in the works, though progress is somewhat slow.
Synapse is much better with regards to resource usage these days, though. The RSS of my instance is 355M right now and the CPU usage is hovering around 0-10% (15-min server load ~0.5).
