"Faster loading pages aren’t just good for people, they’re good for the planet too. The faster a page loads, the less energy it uses. While loading a single page may not seem like a big deal, when you consider that over half the world’s population is online and there are nearly 2 billion websites on the internet, the numbers really start to add up. Plus, very little of the internet is powered by renewable energy, so it is, in effect, the world’s largest coal-powered machine. Faster load times mean less CO2 emissions."
Source: Tim Frick from Mightybytes