I have also been having this issue. I opened a Feedback (FB18350833) but have not heard back. It seems that sometimes it times out, and sometimes it fails early.
I've noticed that if I clean my build folder, restart Xcode, and rebuild then I can usually get it to build the whole project. Once it's built successfully once, then it will continue building. My current issue is getting a TestFlight archive to build correctly.
Topic:
Developer Tools & Services
SubTopic:
Xcode