The Collaborative International Dictionary of English v.0.48:

Swiftness \Swift"ness\, n.
   The quality or state of being swift; speed; quickness;
   celerity; velocity; rapidity; as, the swiftness of a bird;
   the swiftness of a stream; swiftness of descent in a falling
   body; swiftness of thought, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

swiftness
    n 1: a rate (usually rapid) at which something happens; "the
         project advanced with gratifying speed" [syn: speed,
         swiftness, fastness]

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "swiftness":
   abruptness, agility, air speed, alacrity, bat, brevity, briefness,
   briskness, celerity, decisiveness, dispatch, expedition,
   expeditiousness, fastness, feverishness, flight, flit, flurry,
   furiousness, gait, ground speed, haste, hastiness, hurriedness,
   hurry, hustle, immediacy, immediateness, impetuosity,
   impetuousness, impulsiveness, instantaneousness, knots,
   lightning speed, miles per hour, nimbleness, pace, precipitance,
   precipitancy, precipitation, precipitousness, promptitude,
   promptness, punctuality, punctualness, quickness, rapidity,
   rashness, readiness, round pace, rpm, rush, rustle, sharpness,
   shortness, smartness, snappiness, speed, speediness, spryness,
   suddenness, summariness, swift rate, velocity