Juliet sarcastically notes the messenger's breathlessness and complains about the lengthy excuses which are delaying the delivery of his message.
Juliet is questioning why the messenger is panting and out of breath if he has enough breath to say he's out of breath. She points out that the explanation for his delay is actually taking longer than it would have to tell her the actual message he's supposed to be delivering.
In essence, Juliet is expressing her frustration that the messenger's excuses are taking more time than if he had just shared the original news. Her words underscore the urgency and anticipation she feels, as well as the unnecessary prolonging of the information she's eagerly waiting for.
Juliet criticizes the messenger for being out of breath for no substantial reason and making lengthy excuses instead of delivering the message promptly.