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]