Canonman,
First, what shields do you have running - If you have the behavour shield running try (a) turning it Off and (b) uninstalling it. I have uninstalled it on two of my PC's, and for about 95% of the time, my internet performance using either IE7 or Firefox and with the web shield on and using the default values is quite good. In fact there is almost no significant difference when the web shield is turned off and speed tests are carried out.
For the other 5% of the time, and at this stage virtually at random during normal browsing, internet speed with either IE7 or Firefox becomes really slow, and it doesn't seem to be on any particular site. If anything Firefox seems to be affected more than IE7. Access to the same site at the same time from another computer running Avast 4.8 via a common router is quite normal.
Restarting the browser doesn't seem to help. When this problem occurs turning off the Web shield completely does appear to overcome the problem, and turning it back on most often brings the problem back (although some times restarting the web shield cuts off access to the internet altogether and then requires a reboot). In any event a reboot without changing the shields also seems to remove the problem.
At this stage it appears that in my case turning off "use intelligent stream scanning" reduces the extent of the problem, however because the system can work for 95% of the time with no significant impact on performance, the problem is not going to be easy to find. (I may be on the wrong track altogether but I wish I could see what is happening to packet retry level when the problem is present)
One thing that you might look at - the web shield is processing data in realtime, and there might be some clash or interaction if you have other software also providing real time protection.
Sorry I can't be of more help.