Updated to 10.3.7 on my 15' alBook and suffered same slow Internet performance that is widely reported on the web. It was especially terrible with my AirPort at home.
The suggestion to make sure you load 'valid' DNS numbers was the only tip I needed. I loaded a different set of DNS numbers and ZOOM it took off. Then I put back the original DNS numbers and ZOOM it keeps on rocking... my previous speed had returned. It was not a new 'valid' number it needed, but simply a reload of the DNS addresses to correct some type of corruption. Even thought they are correct, try erasing, then reloading your DNS addresses and see if it works for you.
Something about 10.3.7 messes up the DNS server lookup process. But it is easily fixed in the Network Preferences panel.
This feels credible. A problem with a .plist file is a typical OS X glitch. Deleting the pref file for network preferences might work as well.