Slow DNS resolution.
For the last several months (probably since some OS X update), I’ve had this bizarre thing going on with my iMac where DNS queries take a ridiculously long time to complete. Web browsing is painful, incoming ssh connections are punished (lots of reverse lookups), and so on. None of the other Macs under my administrative control have the issue, so I figured it was just something weird about my setup. Then I learned that this is one of those widespread problems Apple won’t own up to [link to one of many Apple forum threads on the topic]. Fan-fracking-tastic.