In the meantime I found evidence that the blur effect indeed stresses the graphics stack. It's an effect the requires fully programmable shaders:
unfortunatly more information is only available in german here:
and unfortunately it seems KDE core developers only care about the 3D stack they use and effects don't get tested on different hardware (intel, nVidia, ATI) and software (Mesa based, closed source OpenGL)
The KDE community has plans to utilize OpenGL 3.x effects:
The code paths in the 3D stack that KDE is using seem to be pretty unique and maybe unsuitable. It's not the first time effects slow down the entire system to unusable framerates / screen refreshes:
I have the feeling KDE lacks a lot of optimization with everything that's related to compositing and usage of 3D hardware/software.