The person's reason to read or who asked the person to read doesn't really affect their skills. But the person asking might make them nervous, which affects one's ability to do something well. Also, the place can affect if its noisy or uncomfortable. It is also important to notice that the more you read, the better you read. Which makes it easier to read difficult texts, since they can affect you as well.
The infinitive phrase in this sentence is to speak clearly, and it is used as a subject of the sentence. What/who is essential to being understood? To speak clearly.
