"Sustainability" is the opposite of efficiency. To be sustainable we have to consume less and stop growing. "Efficiency" just empowers us to eat the planet faster.

Internalize the costs of energy as a first step. If you manage to make web fonts cost $2 per load, people will find their own ways to use less of them. If you make web fonts CHEAPER to load by making them "more efficient," then people will use MORE of them!