As Iray preview render uses screen resolution, it becomes difficult to use on a 40 inch 4K screen (3840x2160 is recomended settings).
This despite the fact that I reduced GI bounces two 8.
Would not like to reduce the resolution for all other programs.
The emergency solution right now is to reduce the CC3 window to half on the screen.
p/s Using both GTX1080 and GTX1080 on my computer.

Hope it is possible to improve this, especially when iClone Iray arrieves..