"Algebraist" by Ian Banks is about human exploring a civilization that lives orders of magnitude slower than us. For them universe is easy to colonize because they live so slow that interstellar travel is fast.

We have things on Earth that live very slowly or pause metabolism altogether for long periods of time. It's not a big stretch to imagine a civilization that can do this (and started very early in universe age) that colonized a lot of universe despite speed of light constraint.