If you don't let her breathe, she can't squirm, giggle, or stammer out cute, desperate pleadings for mercy for you to interrupt by forcing squeals and bursts of hysterical laughter. All that sexy squirming about takes energy, too. She can't escape, and you've got all the time in the world to do whatever you please. She feels that, over every inch of her cute, nubile, fit little body...and that is what a good tickle-torturer should play upon - accentuating her helplessness, showcasing your ability to produce all sorts of amazing bursts of unbearable pleasure about her flesh at your whim, and enjoying the sexiness of her hysterical antics. Prolonging that requires pacing.