Synonyms for Slowness (same or very similar meaning)
WordNet sense 1 (unskillfulness resulting from a lack of training):
awkwardness, clumsiness, ineptitude, ineptness, maladroitnessWordNet sense 2 (lack of normal development of intellectual capacities):
backwardness, retardation, subnormality, mental retardationWordNet sense 3 (a rate demonstrating an absence of haste or hurry):
deliberateness, deliberation, unhurriednessFrom the ODE community, based on WordNetadd/edit