Well, here’s something I hadn’t spotted before, and in retrospect it makes a lot of sense: Microsoft introduced a cap on the number of SYN (half-open connections) for TCPIP.SYS to 10 in XPSP2. Yes, that would explain the slower P2P and Fasterfox browsing then…

David Kaspar’s blog post describes the symptoms, how to demonstrate that there is a problem using the event log, and links to a patcher which raises the cap to 50 connections. The tone of the post starts off a little hystericaly, but settles down into a good description of the issue and the fix.