Which is the slowest-moving mammal?

The sloth moves more slowly than any other mammal, and the reason is arithmetic rather than laziness. Its diet is tree leaves, which are low in energy, hard to digest and often mildly toxic, so a sloth extracts very little fuel from a large volume of food and takes a long time doing it, sometimes weeks to pass a single meal. With that budget, speed is unaffordable. Everything else follows: a very low metabolic rate, a body temperature that drifts with the surroundings in a way unusual for a mammal, greatly reduced muscle mass, and long curved claws that let it hang from a branch by hooking rather than gripping, which costs almost nothing to maintain. Slowness also works as camouflage, since predators such as eagles and cats hunt largely by detecting movement, and algae growing in the grooved hair gives the coat a greenish tint that helps further.

Sloths swim considerably faster than they climb, using the same long-armed stroke to far better effect in water.