This has also been happening to me and only started happening recently.
From my observations, it is not due to anything local but rather App Store seems to be causing downloading several tens of megabytes of stuff from Apple web sites (developer.apple.com, among others) and also seems to involve the program ocspd.
Apple, we await a fix from you....
This is odd, I created a new user and tried to access the app. store.
Worked great, with no waiting time, as snappy as I was used to.
unfortunately my admin account still has the same problems that I stated earlier
Install the v10.6.7 combo update. It has MAS bug fixes that may help.
It's ok to do this even if you are already running v10.6.7
Restart your Mac after the update is installed.
I found some help at the discussion below, in the first posting by smartpuppy:
See a possible remedy here: