Are you refering to buffering? If this is the case, this used to happen to me when I had WinXP. To get round it, click 'Play' on the video and when the little round icon starts moving from left to right, hit the pause button. The icon will freeze, but you should see another lighter coloured line moving from behind it. Wait until it gets about half way across. Then hit the play button.
iMac Pro review