There are many synonyms for the phrase "with breath-taking speed." Some possibilities include "lightning fast," "blindingly quick," "astonishingly rapid," "shockingly swift," "unbelievably speedy," "impressively brisk," "startlingly expeditious," "jaw-droppingly hasty," "staggeringly fleet-footed," and "mind-bogglingly accelerated." Each of these phrases conveys the sense of something happening incredibly quickly, leaving observers gasping in amazement or shock. Whether describing a racehorse, a sports car, or a computer program, these synonyms for "with breath-taking speed" are sure to impress and captivate anyone who hears them.