I've had problems with stuck updates, or slow downloads with the MAS for as far as I can remember.
softwareupdate never really was of much help, using MAS'
Debug menu neither, nor killing
softwareupdate related processes.
Yesterday I found the answer to all of this: a gem for manipulating the MAS from the CLI. It uses native APIs, from login (the MAS login pops up), to downloading files (you can even start an update with the MAS and finish it on the CLI - the download files are the same). And you also get a nice, little progress bar.
A few commands available:
$ mas list 446107677 Screens 407963104 Pixelmator 497799835 Xcode $ mas install 497799835 ==> Downloading Xcode ==> Installed Xcode $ mas upgrade Upgrading 2 outdated applications: Xcode (7.0), Screens VNC - Access Your Computer From Anywhere (3.6.7) ==> Downloading Xcode ==> Installed Xcode ==> Downloading Screens ==> Installed Screens
I'm quite happy.