Ultraslow is an adjective that describes something that is extremely slow or sluggish. Its antonyms, or words that have the opposite meaning, include fast, quick, rapid, swift, speedy, and accelerated. These words describe something that moves or operates at a high speed or frequency, and are often associated with efficiency and productivity. Some other antonyms of ultraslow are brisk, hasty, hurried, expeditious, prompt and expeditious. A good example of using antonyms for ultraslow in a sentence would be, "The tortoise moves ultraslow, but the hare is incredibly fast.