In this paper we describe the Medusa proxy, a tool for exploring user-perceived Web performance. The Medusa proxy is a non-caching forwarding proxy used in conjunction with a user's browser. The two key features that the Medusa proxy provides are (1) the ability to simultaneously mirror HTTP requests from the browser to different Web delivery systems and directly compare the results, and (2) the ability to transform requests, e.g., to transform Akamai URLs back into URLs to the customer's origin server.