It's worth noting that this sounds to be an unfederated server. The reason Synapse has a reputation for being resource hungry is on publicly federated servers:

It's worth noting that this sounds to be an unfederated server. The reason Synapse has a reputation for being resource hungry is on publicly federated servers: if one of the users starts going and joining a bunch of large busy public rooms from their new server, then those rooms get dutifully replicated onto the server, which inevitably consumes resources. The more users and the more servers in the room, the more changes of gnarly forks and merge resolution problems, and the more the risk of CPU spikes. We're constantly working on the memory footprint and state resolution algorithm (e.g. https://github.com/matrix-org/synapse/blob/develop/docs/auth_chain_difference_algorithm.md https://github.com/matrix-org/synapse/blob/develop/docs/auth... landed a few days ago), so the situation is improving, but this is the root cause.