When you have the menu (or the inventory) open, you don't see any difference between 30 and 60 FPS: in fact, lower framerate is visible especially when you move the camera.

My "benchmark": standing still at different framerates
CPU GPU
30fps 8% 11,5%
60fps 13% 20%

I could see this making sense, but we'd have to slow down the rendering of only the world, not the GUI screens. I think it'd be annoying if e.g. dragging items had only half the FPS. What do you think?