In our lab we use a VTK8.2/Qt5 based application on about 15 computers but only observed this delayed update (you need to move the mouse or move to another window to get the widget updated) on only one of them. It happens not just for the VTK render window but for other standard Qt widgets, too. If I restart the computer then usually the problem goes away.
What computer do you have this problem on? Is it a laptop/desktop? What is the operating system and graphics card? Can you reproduce it on another computer, too?