Well, it turns out the problem is a lot simpler (and considerably less nefarious): It’s related to the AV/firewall suite that Cox license from Authentium. Since we do business with Authentium, I thought I would contact them to get an explanation.
Here’s what happened: Craigslist does the unusual step of sending a TCP packet with a zero-length window (typically used to indicate that that the server is experiencing congestion and can’t handle more data).
Those subscribers running the suite will experience delays in getting to the site, since Authentium’s firewall responds to the zero-length window by sending data only one byte at a time, even after the server increases the TCP window size.
Authentium is on it and is fixing the problem.
The whole thing is explained here by Ray Dickenson of Authentium.