I had a similar problem which I discussed here https://community.jboss.org/thread/154296?tstart=0
My solution was to break the page into separate iframes. That way the browser page downloads, renders and is responsive to the user while each of the iframes loads in parallel.
Can you explain what do you mean by "different sessions"?
My parent page and child page both bind to the same session-scoped bean which is only instantiated once.
I guess, I just like iframes. But then again I always liked frames even when they weren't fashionable