I figured it out and it is not fixable.
It is slower because of a fix for exception handling. So messages between actors sharing the same mailbox are just a bit faster than messages between actors with different mailboxes. But if you want speed and the mailbox is shared, you can always just do a method call.
Messages passed between mailboxes are wrapped to track additional information. Now all messages are wrapped and all requests subclass Request. If we create a Response class and require all responses to subclass it, then we could eliminate the wrapper and its subsequent garbage collection. Perhaps that could be a part of JActor release 4.
Bill