Sloths move slowly because of their herbivorous diet, slow metabolism and small muscle mass. Sloths are able to move fast, but they have to burn an enormous amount of energy in order to do so. The sloth diet lacks fats and proteins, which are important sources of energy and nutrition.
A sloth's metabolic rate is nearly 40 to 45 percent that of similar-sized animals. Humans need less than a day to digest a meal, while sloths need up to a month. This slow release of nutrients means that sloths have less energy to burn and therefore need to move slower in order to conserve what energy they have. Sloths' slow metabolism also means that the animals sleep for long periods of time. Sloths sleep up to 10 hours a day while in the wild and as much as 14 hours per day while in captivity.
Sloths also have half the muscle mass of other similarly-sized animals. On the ground, sloths move by dragging themselves with their long arms. They are unable to walk upright because what muscle mass they have is mainly located in their arms and torso. This technique of moving from place to place also causes them to move at a slow pace.