How much memory is available on the machine you installed Wowza?
Have you looked at these links?:
General Tuning
https://www.wowza.com/docs/how-to-do-performance-tuning&highlight=java+version
More Tuning, Running the Server Java VM
http://community.wowza.com/t/-/41 (windows)
http://community.wowza.com/t/-/44 (linux)
Richard