I’m not running a Synapse server myself so I can only speak on behalf of people I know who are. From what they told me they love the matrix protocol but it’s not the same for the synapse implementation. A non-federated server can have somewhat great performance but a federated one was not worth it for them so they decided to switch to another alternative. They are not running for thousands of users more something like 40 I would say and while I don’t know their server specs, I assume it’s not a potato though.