Having trouble after an system upgrade from Leap 42.2 to Leap 42.3
I remember having this problem before, but not the solution I did to overcome it.

I do not have Virtualbox installed.
No optimus / bumblebee.
Nouveau is blacklisted.

When KDE is loading it takes longer, a lot longer.
It looks like it paints in layers and it takes a long time before a window is "painted"/drawed.
Mouse is sluggish after/when I have clicked on a window to close, when I move over icons.
Monitor is flickering.
Typing is a pain.
Second monitor will not activate.

The problem: NVIDIA was not loaded.

I first started to uninstall the previous kernel (which came with Leap 42.2)
Then reinstalled NVIDIA.
During this install it had some warnings/errors:
Installing nvidia-gfxG04-kmp.......
depmod: WARNING: could not open /lib/modules/4.4.76-1-default/modules.order: No such file or directory
grep: /etc/sysconfig/kernel: No such file or directory
Installing x11-video-nvidiaG04........
grep etc/Xorg/xorg.conf: No such file or directoy
I deleted several of the old kernels under /lib/modules
Reinstalled kernel
Reinstalled NVIDIA
It still complains about the same thing.

Restart and everything looks fine. NVIDIA is loaded.
System is fast again.
Not sure what did the trick and got my system working again.

Seems there is some housekeeping needed after an system upgrade. Just one day I want a system upgrade to go without problems (is it too much to ask for?).