Minimal response time for client-server application.

Good evening guys...

I used to use HttpClient to execute remote call from a client to a remote application server. The whole thing works, but currently I'm investigating on how much I can get the system go faster, so I'm playing around with different approaches to execute RPC. Since both client and server use Java (the server is a Java EE appserver), I don't care about using different languages. The problem I'm trying to solve is to determinate if exists a communication protocol which minimizes request/response cycle time, or, at least, to calculate what would be a lower bound for a client/server communication, given bandwith and /or RTT (obtained by a ping for example).

Can you address me some starting point ?
Thanks in advance !