I don't use IE8, or 7 for that matter, and don't have a definitive answer for you, but a couple of members on here found their entire PC seriously slowed right down after updating to IE8, and the problem went away after reverting to IE7.
IE8 is causing various slowdown problems all over the internet, and apparently Microsoft are aware of this. If it was my PC I wouldn't even attempt to find another reason for your problem, I would instantly revert to IE7 until IE8 is sorted by MS.
I've got Windows Vista Home Premium and Internet Explorer 8 and CCleaner 2.20.920.
Usually I use Firefox, but I visited 4 Webpages and then ran CCleaner, it took "1.941 Sek" to clean the PC for 1,67 MB in Gutmann-mode - I'll use IE a bit more and report my results.