Some trees -- mostly evergreen conifers like pines or spruce -- use solely this technique and keep their needle-like leaves unfrozen year-round with a super strong anti-freeze.
We were much faster than our own hair, which would have grown a meager 27 km in the 180,000 years we took to cross Africa and reach all the unfrozen continents.
After a while, when Harry had unfrozen three times in a row, he had Neville join Ron and Hermione again so that he could walk around the room and watch the others.