Personally I wouldn't want to disable the webshield component. Avast scores very highly for prevention, and well for removal. My point being that prevention is better than cure, and the webshield a significant part of that prevention.
There must be something else at play; the presence of the webshield doesn't seem to affect my browsing speed perceptibly.
System, RAM and CPU might be factors.
Pre-loaded applications that might also be connecting might be factors, I guess. Depends what has been loaded at the factory, and how that might interface with the web.
PCDecrapifier might be some use, but it's totally a stab in the dark.
You might want to consider posting a HijackThis log, it might give the experts here (or even me) an idea of what is at play.