I have exactly same problems on a new MacBookPro 2015 i7 quadcore and 16 gb of ram, it is unbelievable.
I spent more than 2500 € for a laptop and I cannot use it with Cubase.
Also, I have to say, Steinberg assistance is near to 0, you have to wait 2 months to have a reply by email, so I suppose I have to close my studio for 2 months waiting for a Steinberg reply?
I really think this will be the right time to switch to another software.
Anyway if someone can help will be appreciated.
Thanks