Some things to try when you have network performance problems in Linux.

Symptoms: On some (not necessarily all) websites, you'll see a delay of several seconds on the “Looking up…” message in Firefox.

I first tried the obvious step of checking/changing my DNS server addresses, but this didn't help. I resolved the issue by changing the “hosts” line in /etc/nsswitch.conf.

#hosts:          files mdns4_minimal [NOTFOUND=return] dns  # original line
hosts:          files dns  # changed to this