Google toolbar shouldn't cause slow-downs, but I am surprised when you say that is the only add-on. How do you surf without Java , Adobe Shockwave and Flash? I'm assuming you do in fact have those.
Anyway, you say that disabling the firewall cures it, so let's make sure that firewall settings are OK.
Double-click the taskbar icon to open Security Center
Click Advanced Menu (bottom left)
Click Configure (left)
Click Internet & Network (top left)
Click Advanced (right)
Make sure the protection level is set to automatic or at the most Standard. Anything tighter will cause slow-downs anyway.
Go to System Service (left) and make sure that at least Common System Ports is checked, if not then check it and click Apply/OK.
Go to Program Permissions (left) and make sure in the right hand pane that Internet Explorer has Full Access, if it hasn't then click "Allow Access" (below-right) and click Apply/OK.
While you are in there make sure that all McAfee Services are allowed full access too.
Just to be doubly sure check out Windows Firewall and make sure it's disabled.