In the play The Merchant of Venice, how does Shakespeare create suspense in Act 3, Scene 4?

In Act 3, Scene 4, Portia tells Lorenzo that he is in charge of her household until her husband returns. Portia then tells Lorenzo that she and Nerissa will be praying and contemplating alone in a monastery. When Lorenzo leaves, Portia immediately tells Balthazar to deliver an urgent letter to Doctor Bellario without stopping on his way. Portia then explains to Nerissa that their husbands will see them before they think, but will not recognize them. Portia goes on to comment that she will act like a gentlemen and tell numerous lies to make her audience believe that she has just left school. Just before the scene ends, Portia tells Nerissa that she will explain her entire plan during the coach ride and mentions that they must travel twenty miles in one day.
Shakespeare creates suspense by making the audience aware that significant events are immediately taking place, while not revealing Portia's entire plan. The audience knows from Portia's insistence that Balthazar urgently deliver her letter that its contents are important to the plot. However, Shakespeare creates suspense by not revealing what was in the letter. Shakespeare also does not reveal Portia's plan, which leaves the audience wondering what will happen next.