I had the problem some time ago. Do you have anti-virus software installed? In my case it was Sophos, that tried to check the jar, while sbt was still building it, which somehow led to 100% CPU usage and the assembly task taking forever. Check your ttrockstars activity monitor for task other than java using large amounts of cpu time and try to deactivate them. I don't know if there is a way to keep Sophos from checking the jars, would be interesting though.
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