All client queues have a retrieval cursor. This cursor speeds up access to messages when there are a large number of unacknowleged messages in the queue that are not candidates for retrieval.

In situations where there are a large number of unacknowledged messages in the queue (such as might happen with JMS clients that control client acknowledgment), it is possible for messages to be inserted into the client queue ahead of the retrieval cursor, regardless of their priority.

To remedy this situation, Broker resets the retrieval cursor back to the beginning of the client queue to re-evaluate all messages in the queue after an insert operation is made. If the number of messages in the queue is small, the impact of the reset operation on performance is negligible; however, if the number of messages is large, there could be noticeable effect on performance. Therefore, if your client queues maintain a large number of unacknowledged messages, consider turning priority messaging off if you notice an unacceptable impact on performance.