A customer noted that when the nonBlockingRedelivery flag was set on their transacted clients, performance would fall off dramatically under a load. Other clients on the same broker are relatively unaffected. After watching the acknowledgement pattern and discussing the issue with Gary Tully, the probable cause of the slowdown was identified as client waiting on responses to ACK, as the client sends an ACK for every message and is currently sending them synchronously:https://github.com/apache/activemq/blob/d6682e5476cd8cbefca04227ffa26a5d508d2494/activemq-client/src/main/java/org/apache/activemq/ActiveMQMessageConsumer.java#L927