same issue here: Big Sessions become very slow and laggy on 12 Core 3,33 ghz mac pro 10.12 Sierra on C9.
Also the CPU is the problem in my case - reaching 1500% usage. same session on windows 6 Core CPU is at 10% CPU usage.
Next test on macpro cubase vs logic : logic can load soooo much more instrument tracks. Cubase gets stucked at 95 tracks. Suddenly freezes and CPU raises from 200% to 1500% again.
After all ive read my conclusion is: cubase cannot handle those 12 Core CPU´s.
I wonder how that is on the new Macpro with newer CPU´s.