When KDE desktop is being started, there's a lot of flicker and other glitches.
After the startup is completed, the desktop may look normal or not. But if you move mouse over the task bar, use the applications menu, or start an application, the flickering continues. The startup splash screen often appears when this problem occurs.

A workaround is to disable compositing. When compositing is off, this problem does not occur. Also, this problem does not occur with Wayland.

Here's a video showing this problem:
https://mega.nz/#!4pQw1AgR!Ujblms_CSSIrczLKpRapoLBiZGrM8xwXJ3JZm7-1EG8


Compositing options:
Enable compositor on startup: yes
Animation speed: medium
Scale method: Accurate
Rendering backend: OpenGL 2.0
Tearing prevention: Full screen repaints
Keep window thumbnails: Only for shown windows
Allow applications to block compositing: yes


System information:
Arch Linux, x86-64, kernel version: 4.20.0-arch1-1-ARCH
Plasmashell version: 5.14.5 (also occurred with 5.14.4)
GPU: Nvidia GTX 980 Ti
GPU driver: Nouveau
KDE workspace themes: 
--Look and feel: Oxygen
--Desktop theme: Oxygen
--Cursor theme: Oxygen white
--Splash screen: Breeze