A default installation of Outlook (or Exchange client) comes with all available protocols.  The unused protocols dramatically decrease performance.  If you are only using TCP-IP you may be able to significantly improve your Outlook performance by making a small change to your Windows registry.  Please observe normal registry precautions when making any registry modifications!

My Computer

    \HKEY_LOCAL_MACHINE

        \SOFTWARE

            \Microsoft

                \Exchange

                    \Exchange Provider

                        \Rpc_Binding_Order = "ncacn_ip_tcp,ncacn_np"