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.