App Store is insanely slow on Mac OS X Lion
I'm not aware of a specific problem with this, but generally when I have an issue of an App running slow, I blame corrupt cache/preference files. What I would recommend is using an App like AppZapper. You can use a free trail from: http://www.appzapper.com/downloads/appzapper.dmg which will last for 10 "zaps"

I would recommend you drag App Store.app into AppZapper, and delete all files BUT App Store.app. Do so by UNCHECKING THE BOX beside App Store.app. On the next launch of the Mac App Store, it'll recreate these (possible corrupt) preference/cache files and hopefully fix your slowness problem :)