34 synonyms for rapid such as breakneck, abrupt, and alacritous. Rapid is an adjective, and its antonyms are slow and leisurely.
Abrupt, alacritous, breakneck, brisk, expeditious, express, fast, fast-track, fleet, fleeting, flying, hasty, headlong, high-speed, hurried, immediate, instant, instantaneous, lightning, lively, meteoric, overnight, p.d.q. (pretty damn quick), precipitate, prompt, quick, sharp, speedy, sudden, swift, volant, whirlwind, and whistle-stop.