My solution is to turn off the proxy. If you have a relative fast ISP-connection and do not pay by byte, proxy is not essential. I have had this trouble on several (actually all) installs, and all is fixed just by turning off the proxy. I do not know what the fault is, but there is nothing wrong we have done, for sure. We are talking fresh installs on rather powerful xeon-servers with lines from 80/80mbps up to 500/500mbps. With proxy it took ages to load even simple pages, and picture-heavy pages could take a minute to load. Without proxy it takes <1 second. I tried some tuning of proxy on one of the servers, but there was minimal difference. I cannot say anything but "something" has happened with proxy that makes it useless. In older versions proxy almost never gave the slightest hickup, it was very stable and on slower internetlines a very good help. On a school we had 250 concurrently used PC's sharing a 2mbps line. With proxy it worked pretty OK, without several complained about timeouts etc. But this is some 15 years ago :-P

Have you seen this article about tuning the proxy?

Ever figure this out?