Who is pedro most likely addressing with this command?

¡hable más lentamente, por favor!

Pedro is most likely addressing to:

- The Personal Pronoun "you".

Explanation:

In Spanish is actually usual finding a sentence that hasn't the substantive or personal pronoun, it occurs because the verbs or the sense of the sentence allows identifying the noun, this act is named tacit subject, in the case of the sentence given, the noun is tacit but by the sense of the conversation in that you can define that the person is "you".