I don't find that a problem with firefox and web shield (not using script shied {home version}), do you have a URL we can try ?
What browser are you using, have you tried others ?
What is the sensitivity in Web Shield, you could try lowering it ?
What do these timeouts look like and what initiates them ?
It could be the web page design, if images aren't given place holders (a size the image will occupy), then the page loading is considerably slowed as it has to load the image before text, etc.