Some of you may have noticed that dragging an inactive window starts very choppily and stutters with Kwin effects enabled even on powerful hardware.
That is because the Active/Inactive switch glow animation does always initiate and for some reason Kwin is fully overwhelmed with doing two things (Dragging and Glowing) at one time

The solution is to get into the Workspace Appearance settings, klick on the Configure Decoration... button, show advanced configuration options and turn off windows active state change animations in the animations tab.
This will also improve the performance of the Present Windows and Workspace Grid effect if you have ever wondered why ending the effect after clicking on an inactive window runs so slowly and stuttering.

Unfortunately you can only disable the animations for the Oxygen Window Decorations so I hope that there will be fix in Kwin for the whole thing.

You can follow the according discussion here
viewtopic.php?f=111&t=87901

- Björn