By design, the performance of the cluster cannot be higher than the performance of the slowest node on the cluster. Even if you have one node only, its performance can be considerably lower when compared with running the same server in a standalone mode (without a wsrep provider). This is particularly true for big transactions even if they were within the transaction size limits. This is why it is important to be able to detect a slow node on the cluster.
The slowest cluster node will have the highest values for the following variables:
wsrep_flow_control_sent
and:
wsrep_local_recv_queue_avg
The lower the values are the better.