Yesterday my browsers became ridiculously slow. Running "ping" or "host" from Konsole showed that the problem wasn't networking.
I found a few posts suggesting that I disable IPv6 and reboot. I created a file "/etc/modprobe.d/bad_list", added "alias net-pf-10 off" and rebooted. Much better.
I was on another machine and don't have links to the actual posts from Linux forums and Ubuntu's forums. My apologies to the solution's authors.