I experienced this locally while I (my user) accidentally didn't have video-card privileges. Using llvmpipe, which I eventually realized, gnome-shell would frequently use more than half of my quad core Sandy Bridge i7 CPUs. For reference, my screen is 1920x1080 (about twice as many pixels as 1366x768). That was enough CPU to make things a bit laggy and make me debug the problem. I'm kind of impressed that Linux was silently so resilient to that permissions problem that I didn't even know anything was wrong at first. How do systems with weaker CPUs manage?