Monitor. 4k lag problem

i’m on Mac Pro 5,1, Sierra, for graphics i’m using an nvidia gtx 660 and i just buy a 27" 4k monitor dell p2715q. So now I have graphic problem whit cubase 9 i do a lag when i drag and drop audio regions. I tried in Windows bootcamp whit same setup and cubase works very well whitout problem …is it normal?.. i would like to know If i buy one gtx 980 can i solve this problem? Thanks in advance