Like Software Update before it, the Mac App Store in Mountain Lion has various caches that are supposed to speed it up. However, sometimes these get corrupted, making it slower instead of faster. If this is happening to you, deleting the Mac App Store's caches will fix it. The tricky part is finding them, though - one is in ~/Library/Caches, and another one lives in a subfolder of /var/folders (took me some searching to track that one down). Both of these locations are hidden in the Finder, making getting to them awkward.