I implemented a web application using Seam, the major functionalities implemented in Session Bean Stateful.
I experienced a major performance issue, when one user do a large upload task, it will block other user login activity. My understanding is JBoss will instance multi-instance of that EJB to handle more request.
I'm not sure anyone can advise you. You don't specify what hardware you're running, how much bandwidth you have coming into the server, what you're doing with the upload. Heck what do you mean by big upload? 1M? 1G? 10G? 1T?
What's the JBoss setup?
There's a lot of questions to be answered before someone can answer your question.