Big delay on COMMIT after third node added

Hi,
I have cluster with 2 nodes in two different places with ping latency about 15 ms. It is working good, the execution time of commit queries is about 15 - 20 ms.
But when I add galera arbitrator as a third node to cluster, which runs on one of the cluster nodes, the execution time of commits is sometimes about 150 - 350 ms. Can you please tell me, what could be wrong?
Thank you.

An arbitrator also has to acknowledge every transaction, even though it doesn’t really use it. So it’s distance to the writer also affects commit latency.

http://galeracluster.com/documentation-webpages/arbitrator.html