Is it true that MySQL Galera is as slow as the slowest node?
Yes it is. Galera relies on group communication between nodes. For any matters, it will wait for all nodes to return the status of certification test before proceed with committing or rollbacking. At this phase, an overloaded node will surely facing a hard time to reply within the time manner, delaying the rest of the cluster to wait for it. Therefore it is recommended to have uniform servers across the cluster. Also, transaction latency is no shorter than the RTT (round trip time) to the slowest node.