The Download of Apps from the Mac App Store is very slow
To download with normal speed again, you can do the following trick:

(These first two steps weren't necessary for me and it worked for me without doing that.)


First open System Preferences → Network → Advanced and choose the tab "DNS" and add another DNS-Server by clicking on the +. A list of free DNS-Servers can be found at about.com. 
Now we choose a third Level DNS (Primary DNS 209.244.0.3, Secondary DNS 209.244.0.4)
Now we have to flush the DNS-Cache of the system, that OSX can use the new DNS-Servers.


Command under Mavericks:

sudo dscacheutil -flushcache

Command under Mountain Lion:

sudo killall -HUP mDNSResponder




The reason this may improve download speeds is that DNS servers may return endpoints to Apple's CDN according to the location they think you are coming from.  If you use a general one (not one from your ISP, or otherwise close to your location) you may get a CDN endpoint that is not optimal for you, resulting in slow/stalled downloads.