Its not IE8, I had S&D on Vista, I had the same problem...
It worked fine until I installed S&D, it slowed after I installed S&D, then it went back to normal after I removed S&D...
Ergo the problem is S&D not IE8.
My solution was to remove the incompatable software.
You did not read the reply carefully. There is a bug in IE8 that if your blocked sites list gets too big it will take up to 30 seconds for IE8 to start. This does not happen in IE7. MS support told me to either downgrade to IE7 or delete the blocked sites list in IE8 until a patch is available. By uninstalling Spybot, you probably cleared the blocked sites list which would fix the problem. Google IE8 and blocked sites list to see ho many are having this issue :)