Since i found that nvidia's forcefullcompositionpipeline does a better job than kwin's tearing prevention in regard of video playback smoothness, i decided to disable kwin's vsync.

But i found that he behaves in a strange way.
My screens (dual head setup) runs at 75hz
If i disable kwin tearing prevention, it seems that it renders at a capped 60fps, indeed i notice that window drag is jerky.
That situation can be corrected by any of the following:
* enabling kwin tearing prevention
* switch the display to 60hz
* setting MaxFPS=75 in kwinrc

Reference:
https://bugs.kde.org/show_bug.cgi?id=372739

Could you please try to reproduce my observations?
The steps would be:
1) switch to a 75hz video mode
2) disable kwin tearing prevention
3) drag a window around and see if the movement is smooth.
Tearing is of course expected, but if kwin is rendering at 75hz, you should see a single "tearing" line.

Thanks.