Learn Polish: Ten nowy głośnik czyści się powoli.

This is the middle voice, which is the focus of this lesson. By adding się to the transitive verb czyścić (to clean), you shift the focus from who is doing the cleaning to how the object experiences the action. It behaves exactly like the English structure where an inanimate object takes an active verb, as in "the speaker cleans slowly" or "the car drives well."

Is Ten nowy głośnik the subject or the object of the sentence?

Grammatically, it is ambiguous because masculine inanimate nouns share the exact same form in both the nominative and accusative cases. It could be the nominative subject (middle voice: "This speaker cleans slowly") or the accusative object of an impersonal sentence (like Książkę in Książkę czyta się łatwo). For Polish speakers, the practical meaning is identical either way.

Why do we use the imperfective verb czyści instead of the perfective wyczyści?

Middle voice constructions that describe a general, permanent trait of an object almost always use imperfective verbs. You are describing an ongoing characteristic (how it generally cleans). If you used the perfective wyczyści się, it would change the meaning to a specific future event: "it will get completely cleaned slowly."

Why use the adverb powoli instead of an adjective?