I was trying out the Beta build of Kubuntu 18.04 LTS and I faced the same issue I faced earlier with 17.10. When resizing any window, there are times when it will flicker. It's random, and switching compositing backend to openGL 3 didn't help either. Using Xrender helped in removing the flicker, but then everything started to tear. Even when I forced full screen repaints, the tearing wouldn't stop.
I don't know if it's a Xorg issue or something with KDE. I own a hardwared powered by an Intel B920 Pentium chip with no dedicated graphics.
This issue wasn't there in 16.04, and if there's anything I can do to generate more technical report, do let me know.