Antonyms of tardy :
punctual, expeditious, prompt, ready, accelerated, hastened, quickened, hurried, rushed, barreling, bolting, breakneck, breathless, brisk, careering, dizzy, fast, fleet, flying, hasty, hurrying, lightning, meteoric, quick, racing, rapid, rocketing, running, rushing, scooting, scudding, scurrying, snappy, speeding, speedy, swift, warp-speed, whirling, whirlwind, whisking, zipping, opportune, seasonable, timely, early, inopportune, precocious, premature, unseasonable, untimely.
Examples of usage:
It is, but tardy. "Yolanda of Cyprus", Cale Young Rice.
She had read the dismay in his face, had in vain waited for him to speak and no tardy lie would convince her now. "The Shadow of the East", E. M. Hull.
It was, perhaps, a tardy beginning for a career such as his must be: but it was a complete one, at least. "The Genius", Margaret Horton Potter.