The client send a request to a servlet. The servlet validates the request and if valid, sends a response to the user, then invokes the business tier (a stateless session bean). This all happens on a single thread of execution.
If the rate of receipt of requests is faster than the rate at which the business tier processes requests I expect each new request to create a new servlet instance (ie a new thread). The behaviour I see is that new requests are blocked until the current thread has completed.