Synonyms of breakneck

1 Syllable
swift, fleet, quick, fast, steep, pdq
2 Syllables
quick-fire, fasted, parlous, steeper, headlong, high-speed, hastiest, speedy, quickest, fastest, steeping, quicker, swiftest, hastier, hasty, fleetest, asap, rapid, speediest, whirlwind, lightning, swifter, speedier, hurried, chop-chop, reckless, faster, steepest, speedball, fleeter
3 Syllables
rapider, rapid-fire, jeopardous, double-quick, fleet-footed, barrelling, foolhardy, dangerous, fleetfooted, posthaste, double-time, screamin', barreling
4 Syllables
supersonic, hypersonic, excessive, dangersome, super-sonic, alacritous, ultrasonic, declivitous, harefooted, expeditious, nimble-footed
5 Syllables
precipitate
7 Syllables
overventuresome