You've pretty much said it all, my friend. KDE is a steaming bucket of cack, I would thoroughly expect it to be laggy under all kinds of circumstances that a respectable DE wouldn't. There was a nice thread about it on here not too long ago, where Martin tried to convince everyone that the sky is green at night, and red during the day, despite many people claiming to have experienced the cack-ness of Kwin. In fact, Intel IGPs probably provide the best KDE experience, while providing the absolute worst experience in every other DE. You wouldn't even have to turn off the desktop effects if you weren't using KDE, I would invite you to try it with something like the latest Ubuntu and Unity, or even a Gnome2 or 3 distro.
If you experience Lags on intel-systems when running the iGPU, just disable desktop effects (Or use a more performant window manager).