The Collaborative International Dictionary of English v.0.48:

Slowness \Slow"ness\, n.
   The quality or state of being slow.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slowness
    n 1: unskillfulness resulting from a lack of training [syn:
         awkwardness, clumsiness, ineptness, ineptitude,
         maladroitness, slowness]
    2: a rate demonstrating an absence of haste or hurry [syn:
       slowness, deliberation, deliberateness,
       unhurriedness]
    3: lack of normal development of intellectual capacities [syn:
       retardation, mental retardation, backwardness,
       slowness, subnormality]

Moby Thesaurus II by Grady Ward, 1.0:

294 Moby Thesaurus words for "slowness":
   Fabian policy, Micawberism, acedia, afterthought, amble, antipathy,
   apathy, aridity, asininity, averseness, aversion, backwardness,
   barrenness, bind, block, blockage, blockishness, bloodlessness,
   boobishness, boorishness, boredom, bovinity, bureaucratic delay,
   characterlessness, cloddishness, colorlessness, crassness,
   crudeness, cursoriness, deadness, delay, delayage,
   delayed reaction, deliberateness, deliberation, denseness, density,
   detention, dilatoriness, dim-wittedness, dimness, disagreement,
   disinclination, dismalness, disobedience, disrelish, dissent,
   distaste, do-nothingness, dolce far niente, doltishness,
   double take, drag, dragginess, dragging, dreariness, droop,
   drowsiness, dryness, dull-wittedness, dullardism, dullness,
   dumbness, duncery, dustiness, effeteness, emptiness, enervation,
   ennui, ergophobia, etiolation, faineancy, faineantise, fatigue,
   flatness, flounce, foot-dragging, fractiousness, gait, gallop,
   grossness, grudging consent, grudgingness, halt, hang-up,
   hastelessness, heaviness, hebetude, hesitation, hindrance, hitch,
   hobble, hoboism, hold-off, holdup, hollowness, inactivity,
   inanimation, inanity, indifference, indisposedness, indisposition,
   indocility, indolence, inertia, inexcitability, inexertion,
   infrequence, infrequency, insipidity, insipidness, interim,
   intractableness, jadedness, jam, jejunity, jog, lack of enthusiasm,
   lack of zeal, lackadaisicalness, lag, laggardness, lagging,
   languidness, languishment, languor, languorousness, lassitude,
   laxness, laziness, leadenness, leisureliness, lenitude, lentor,
   lethargy, lifelessness, limp, listlessness, lock step, logjam,
   lotus-eating, loutishness, lowness of spirit, lumpishness, lurch,
   mince, mincing steps, moratorium, mugwump, mutinousness, ninnyism,
   nolition, oafdom, oafishness, obstinacy, obstruction, obtuseness,
   occasionalness, opacity, opposition, oscitancy, pace, paddle,
   paleness, pallor, paperasserie, passivity, pause, perfunctoriness,
   phlegm, piaffer, pointlessness, pokiness, ponderousness, prance,
   procrastination, rack, rareness, rarity, recalcitrance,
   recalcitrancy, red tape, red-tapeism, red-tapery, refractoriness,
   refusal, relaxedness, reluctance, remissness, renitence, renitency,
   reprieve, repugnance, resistance, respite, retardance, retardation,
   roll, satedness, saunter, scarceness, scarcity, scuttle,
   seldomness, shamble, shiftlessness, shuffle, sidle,
   simpletonianism, single-foot, slackness, sleepiness, slink,
   slither, sloth, slothfulness, slouch, slow-up, slowdown,
   sluggishness, solemnity, somnolence, sottishness, sparsity,
   spiritlessness, spring fever, stagger, stalk, stall, stay,
   stay of execution, step, sterility, stiffness, stodginess,
   stolidity, stop, stoppage, stride, stroll, strolling gait, strut,
   stubbornness, stuffiness, stupefaction, stupidity, stupor, sulk,
   sulkiness, sulks, sullenness, superficiality, supineness,
   suspension, swagger, swing, tastelessness, tediousness,
   temporization, thick-headedness, thick-wittedness, tie-up,
   time lag, toddle, torpidity, torpidness, torpitude, torpor, totter,
   tread, trot, uncommonness, unenthusiasm, unfrequentness,
   unhastiness, unhurriedness, uninterestingness, uniqueness,
   unliveliness, unteachability, unusualness, unwillingness, vagrancy,
   vapidity, vapidness, velocity, waddle, wait, walk, weariness,
   woodenness, world-weariness, wrongheadedness, yokelism