I was hoping you were going to say Vista and zone alarm as there is a bug in ZA with Vista that slows down browsing and that isn't just with avast. You should only Terminate the Web Shield, rather than all on-access protection, that would at least give the standard shield as a fall back.
Is it ZA Pro or free ?
I'm on a dial-up connection and there is no appreciable difference with the web shield enabled, I have XP Pro and Outpost Pro firewall. So the only difference is our firewalls. However there are many forum members that also use ZA and there is no flood of topics relating to this problem. I'm not saying it is not happening just there is likely to be something else in the mix.
What other security based software do you have that might have an effect ?
Other than AVG, have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?