What does the end of a session mean

We are trying to diagnose a possible performance issue. A WebSphere servlet calls an IS 4.6 service on port 5555 (the main port). They are periodically getting slow responses back from IS, however, we are seeing their session end on an average of 1/2 seconds. We are of the belief that as soon as we see the session end line in the session.log file, that all communication back to the client has completed. Is there any reason why it should be otherwise? The WebSphere servlet is connecting to IS through the Context class. The data transport is left to the default value, RPC_BIN.