I have the very same issue, I'm also on a Mac. The problem started recently, but I can't seem to shake it off at this point. I tried deleting the preferences, deleting color presets and disconnecting external storage like Drobos.
It freezes on startup and won't bounce back until 5 minutes later, including the mouse pointer becoming unresponsive. Cpu usage and memory usage stay moderate during this period. Any suggestions would be much appreciated!