curious architecture here !
So I'll try to help you... No garanty ;))))
I resume :
browsers processes take huge resources on the Terminal Server, and JBoss/Tomcat is sleeepy ? Right ?
So it means :
1 - the browsers have posted there requests to your HTTP server, so they should be waiting to the answers and so wouldn't take resources.
2 - JBoss doesn't take anormal resources... so asynchronous process won' help to do.. nothing !
perhaps there's misconfiguration of thread numbers of IE to allow lot of parallel clients and they are switching context on the Terminal Server indefinitly... or perhaps buggy version of IE...
your browsers are CPU bound in the way they are configured.
you can improve through asynchronicity when the application server is itself overloaded