What a great poem!  I'm always a fan of poems that rhyme.  One thing that is not clear in Roethke's "Sloth" is where the speaker is.  It is not stated if the speaker is actually looking at a sloth right then and there, or if the speaker is reminiscing about the odd qualities of sloths in general.  

Stanza 1: The speaker starts by saying that there isn't another animal out there that can compete with the sloth for slowness, not only slowness of movement, but also in slowness of thought.  "He thinks about it for a year," maybe because the speaker thinks sloths are dumb or maybe because he thinks sloths are simply very deliberate in their decision making process.  

Stanza 2: A contrasting comparison is made between a bird and a sloth.  The bird will sit on top of the branch and the sloth will hang upside down.  The sloth is also described as being quiet or of few words, but assumes that you know what he was thinking or is going to say.  

Stanza 3: That assumption is what makes the sloth so painstakingly "ex-as-per-at-ing."  The third stanza also shows how little the sloth cares for anybody else's opinion.  You could call him names, but the sloth just sighs and stays put.  

Stanza 4: Straightforward stanza. The sloth falls asleep hanging on the branch knowing that he is being a bit of a snot and not caring.