In Act 2 Scene 5, what creates the tremendous tension in this scene from Romeo and Juliet?

Tension in Act II, Scene 5, is created by the Nurse, who seizes the opportunity to be the center of Juliet's attention and exploits this opportunity by teasing Juliet and delaying her answer so that she can have some attention herself. 

As the scene opens, Juliet waits impatiently for the Nurse to return with Romeo's notice about their wedding plans. Since Juliet has sent the Nurse at nine o'clock and now it is twelve, she complains that the nurse is still not back and is dilatory on purpose:

But old folks, many feign as they were dead;
Unwieldy, slow, heavy and pale as lead.
O God, she comes! (2.5.16-18)

But, when she enters, Juliet is further frustrated because the Nurse, taking advantage of being the center of attention, complains that she is tired and her bones hurt. Further, Juliet entreats her, "Is thy news good, or bad?"(2.5.5), but the Nurse again does answer; instead, she needles her about Romeo, saying, 

Well, you have made a simple choice; you know not
how to choose a man: Romeo! no, not he; though his
face be better than any man's....(2.5.38-40)

The silly, loquacious Nurse then speaks circuitously and make puns on decorated shoes and "courtesy/curtsy." And, again she resumes her complaints about her aches and pains. Finally, she gives Juliet the news, but not until she teases Juliet with verbose descriptions of Romeo's handsomeness, kindness, courtesy, virtue--everything but what Juliet wants to hear.

Finally, the Nurse asks Juliet if she has permission to go to confession at the church where Friar Lawrence is now. When Juliet replies that she does, the Nurse instructs her that the priest will perform the marriage ceremony there in the church. Jubilant, Juliet exclaims, "Hie to high fortune!" and happily rushes out.