You know just how hugely powerful an i5 processor is? the "no graphics card" that you actually have inside your CPU is more powerful than the latest gamer graphics card from 5 years ago. And consumes a lot of power too.
Gnome 2 ran quite fast on a 500 Mhz ARMv5, 200 milliWatts idle. Yes, it is a niche application, but it shows just how much hardware (and batteries!) Gnome 3 needs.