How many clients are connected to the database? The cluster controller isn’t involved in the write path, so it shouldn’t be the bottleneck for the write throughput of your database. However, it does maintain a connection to every process in the cluster and every client. Part of its responsibilities are to handle failure monitoring, and the costs for this do add up when you have a lot of clients and/or processes in the cluster. Your number of server process seems ok, so that’s why I’m curious about the clients.
At the moment there is no way to scale out the responsibilities of the cluster controller, although it may be possible to reduce the connection costs with knobs. See Migrating from a large cluster to another for more discussion of this.