I'm investigating this issue more today, I've been emailing back and forth with the Codeplex team. They admit it is slow but say they have tested checkout of mojoportal on a number of machines and it works every time.
I can say that on one of my machines it works every time but on the other it fails every time. This makes me think there is some client machine factor to the problem so I'm trying to figure out what is different about my problem machine vs the other one. I'm going to run Fiddler on the problem machine today while trying to do a checkout and see if the http traffic shows anything different or errors happening.
It is also possible to get the code with Team Explorer, a free tool from Microsoft that can run as a stand alone or a VS 2010 plugin. It works natively with the TFS (Team Foundation Server) repository which is really what we have at Codeplex, svn is made available through a server side svn bridge that translates svn commands into TFS and responds with expected svn responses. So Team Explorer is more reliable, I use it for actually managing the repository merging and branching since the svn bridge is known to not support the branching and merging commands.
The url for connecting with Team Explorer can be found on the source code tab on our Codeplex page. When getting the code, you do only want to get the trunk folder, not the whole repository.