> "I would know not why it should be slow."
This line follows Friar Laurence's revelation that Juliet has drunk the potion he gave her, making her appear dead. He is talking to himself, contemplating why Romeo should take so long to arrive with news about the plan he has devised. He is anxious and impatient, believing Romeo should be acting swiftly to rescue Juliet.
The meaning of the line, then, is:
* Friar Laurence is impatient. He expects Romeo to act immediately, having faith in their plan and knowing the urgency of the situation.
* He sees no reason for delay. He believes the plan is sound and doesn't understand why there would be any reason for Romeo to hesitate.
In the context of the play, the delay, of course, leads to Romeo's tragic misunderstanding and Juliet's eventual demise. This line highlights the tension and urgency of the play's climax, as well as the Friar's hope for a swift resolution.