I think I can confirm this issue on mac as well.
When I run my Mojave (and HighSierra too) 4K60Hz the graphics is much slower than on 4K30Hz. (630 Intel, GT1030 Nvidia and RX580 AMD)
And it seems like the MCU also get a lower update frequency. So it seems that cubase do something weird. They need to look in to that since 120Hz is common this days for game pc’s. It really feels sluggish even if the CPU nor GPU is on high load.