Cubase 8.5 OSX Can not drag audio region smoothly

in Cubase 8.5, i don’t know why everytime i drag long audio region (like full song audio region) , it’s not move smoothly.
you can see in this youtube link i uploaded
https://youtu.be/NXVP3X11sLY

and with the version 8.0.35 on the same system, it’s not happen, i can drag smoothly.

my computer is
Cubase 8.5.0
OSX 10.11.2
iMac (Retina 5K, 27-inch, Late 2015)
4 GHz Intel Core i7
32 GB 1867 MHz DDR3
AMD Radeon R9 M395X 4096 MB