It would seem to me the proxy server is messing up your response times.

Can you do it without the proxy? If not, can you reconfigure the proxy so that requests from your IP will never get cached? Even if you can get it to not cache for you, any data you collect will more or less meaningless as you have no idea what the influence of the proxy server is. Can't you run this script from a machine in your DMZ?

Liz