Totally unscientific: On my ancient Thinkpad T61p with nvidia graphics of some vintage and the proprietary drivers installed on Debian Jessie, I get around 12 to 20% on one of the two cores with vigourous mouse assisted window movement of an Iceweasel window. MATE desktop, compiz is not available in Jessie repos so using marco.

The overall user experience is smooth and pleasant. I doubt if Windows 8/10 would install on this machine (although it came with Windows 7). I have several Thinkpads of this vintage for the keyboards, not needing super powerful processors.

It's because of either different drivers, or your one core is faster than my poor little c2d core, or you're not moving it crazy enough :) Did you compare it with kwin? For example, on my machine gnome shell uses sth. like 40-60% while kwin is 10% at most.

I'm using a 4670K, so there is a difference, but I doubt that this is the explaining factor.

I know that gnome successfully offloads work to the GPU when I drag the windows [1] because I can hear it [2].

I don't have kwin installed, so I didn't compare.

[1] Yes, I also did move them wildly, too.

[2] My graphic cards has serious coil whine issues.