What does the adverb as modify in this sentence?

Solution 1:

To be a bit more precise, englishgrammar explains:

The structure as…as is used to compare things that are of similar proportion.

In this case the first as acts as an adverb modifying the adjective or adverb that goes after it. The second as can act as a preposition or conjunction. If it is used as a preposition, it will be followed by a noun or pronoun. If it is used as a conjunction, it will be followed by a clause.

• He is as cunning as a fox. (Here the first as in this construction modifies the adjective cunning. The second as modifies the noun fox.)
• He drove as fast as he can. (Here the first as modifies the adverb fast and the second as modifies the clause ‘he can’.)

However, Grammarquizzes asserts that the second as is a preposition whether it is followed by a noun or a clause:

The comparative as…as expresses that two items have equivalent aspects. The first as modifiers the quality (Adj) or manner (Adv) of the item being compared. The second as is a connective preposition which is followed by a noun or a clause. If it is a clause, it is shortened to just the subject and the auxiliary form of the verb.

What is definite is that the second as followed by a noun is a preposition. So, to apply this to your example, the second as is used as a preposition having as a prepositional object the noun a sloth.