Cubase pro 8.5 become delay when open video player

I am using Cubase pro 8.5 in win7 64bit,and I have quicktime 7.79 install in my PC

when i import a video(mp4,or mov) into Cubase pro 8.5 project,and open video player to playback the video,then the project can’t work normally,the video can play but the cursor seems delay,and many operations seems delay,like a key comand “G”,"H"to zoom the project seems delay.but when I close the video player,everything become normally.I have try many times,but still have the problem.

I try in Cubase 5,and it seems ok

my pc:
cpu:i 5