Apparently you can have a communications issue due to the version of TCPIP.NLM (5.32y) supplied from the NW50SP6A or NW51SP2A patch which can lead to noticeably slow proxy performance. Here is a setting you can use as a work-around:
SET TCP DELAYED ACK=OFF in your AUTOEXEC.NCF file (after TCPIP is loaded from INITSYS.NCF).
The behavior that you will see is that the proxy requests will go from a few seconds to up to a minute. Using this setting, they will be back down at a few seconds.
If you have been getting a lot of Gateway Timeout errors, this is a setting to try!