the black screen is Muse loading everything into ram and it runs at the speed your system allows i.e, doing things in the background will slow it down and running behind a crappy wifi or any Proxy server will make it even slower... this isn't really a Muse issue mate
But don't you find it odd that it's so slow? Every other CC application runs like the wind on my Mac. I try to be very smart with how I use my computer's resources. That is when I'm working I only have Muse and a browser open. No extra background stuff except for Dropbox and Spotify. Honest. I find it hard to believe that this iMac is not coping well with Muse if everything else from Creative Cloud Suite runs great. Also, this black screen never changes. I left it a few times over night and it was the same in the morning.