yeah, no shit, it's decentralized.
If you don't want that metadata visible then host your own server and require your organization to use only that server, there are settings specifically to enforce this use-case.
the french government uses matrix for communications, it's fine.
synapse is notoriously dogshit because they've ended up needing to constantly focus on developing it rather than finish getting dendrite (the actually optimized server software) feature-complete so it can take over as the reference implementation.
like, jesus christ, synapse is written in python! entirely unsurprising it has terrible performance.