There’s no Trial yet so I can’t try it out myself, but I’m not going to upgrade/continue unless I know that this has been remedied. Would someone who is using it on Mac (this is not a Windows issue) be able let us know? Thanks! 
Better ask in the C9 forum!
What exactly are you referring to? I’ve started to run the program on both platforms, and my early benchmarks indicate Cubase is running as well, if not better, on the Mac than on Windows. Since this is contrary to what I’ve been led to believe from reading forums, maybe you would be willing to explain further what your ‘slow’ Mac GUI issue is.
I went to the C9 forum, but just to address your question: It’s a problem that has been talked about in a very large way by many, many users on this forum. It’s that the GUI in Mac is sluggish and jerky when zooming and scrolling in the inspector especially (more so in projects that have audio and are anything above a small size).
i thought it was a problem that started after the update to el capitan, more like apples problem to fix.
Except that it doesn’t affect any other program I have or use (including Adobe Premiere, Pro Tools, Reaper, etc.).
Worse than 8.5 (if that’s even possible) on both mac and window, especially with lots of audio tracks.
Not happening in ANY other DAW. 