JMS topic deliveries slow down proportionally to the number of subscribers in JBoss Messaging

Issue

• When I use JMS topics to publish/subscribe messages, the deliveries seem to be processed sequentially and can slow down proportionally to the number of subscribers.
• If I have only one subscriber, the delivery is completed in around 10 seconds; if I have a hundred subscribers, it takes 100 seconds to be completed.
• Is there any configuration parameter which allows the parallel deliveries of messages?