Yes sloths do know that they are slow.
Sloths are very slow on the ground because they lack the energy needed to move fast and what little energy the sloth has they want to and need to conserve it.
So the sloth moves slowly to save what little energy they have.
Although sloths can move a bit faster when swimming so they are not always so slow.
Since sloths are so slow it can even take up to a month to digest the food they eat and they slowly come down out of the trees to poop and then slowly climb back up.
Sometimes I feel like moving like a sloth when I can't be motivated or bothered to do anything.
Sloths are slow because of their diet.
Sloths mostly eat leaves, twigs, and flowers that they can easily reach from where they are hanging.
The Sloths herbivorous diet is low in energy and lacks much of the nutrients needed like fats and protein for a balanced meal.