I have had issues on a couple of machines where after upgrading from IE7 to IE8 the installation of IE8 was really slow to load the homepage upon first opening the browser after that the page to page load times were pretty normal.
The first thing I did was to disable all the add ons that load with the browser as I have had those cause issues before but that was not the case here same symptom remained.
I Googled the issue and came across a lot of info on this problem and it seems the main cause is usually related to Spybot Search & Destroy adding entries in the restricted sites list.
It was suggested to open Spybot S&D and click on "immunize" then "undo" which worked for me as soon as I did this IE8 was working just fine so I am passing this info along.
