The Collaborative International Dictionary of English v.0.48:

Rapidly \Rap"id*ly\ (r[a^]p"[i^]d*l[y^]), adv.
   In a rapid manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rapidly
    adv 1: with rapid movements; "he works quickly" [syn: quickly,
           rapidly, speedily, chop-chop, apace] [ant:
           easy, slow, slowly, tardily]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "rapidly":
   always, amain, apace, at all times, at flank speed, at full speed,
   at once, briskly, ceaselessly, constantly, continually, daily,
   daily and hourly, day after day, day and night, double-quick, ever,
   ever and anon, every day, every hour, every moment, expeditiously,
   fast, flat-out, full tilt, hand over fist, hand over hand, hastily,
   hell for leather, hell-bent, hell-bent for election,
   hour after hour, hourly, in a flash, in a jiffy, in a moment,
   in a trice, in a wink, in double time, in double-quick time,
   in high, in high gear, in seven-league boots, incessantly,
   instantaneously, instanter, instantly, lickety-cut, lickety-split,
   like a shot, like blazes, like greased lightning, like mad,
   like one possessed, month after month, night and day, on and on,
   on the double, perennially, perpetually, post, posthaste, promptly,
   quick, quickly, regularly, right along, right away, snappily,
   speedily, steadily, sustainedly, swiftly, trippingly, unceasingly,
   under forced draft, unintermittently, uninterruptedly, unvaryingly,
   whip and spur, with all haste, with giant strides,
   with rapid strides, with speed, without cease, without delay,
   without letup, without stopping, year after year