A solution to decrease server load would be to put the shoutbox on its own page. Remove it from the sidebar. I guess most of the time the users don't use it, as they browse the forums. When they like to chat they can go to the shoutbox page and chat with other members.
Or make the shoutbox a popup window. The users who like to chat can leave it open.
In both cases performance is saved and the constant reloads are only produced by the users who really use the shoutbox.
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