WordNet sense 1 (any agent that retards or delays or hinders):

retardant, retardent

WordNet sense 2 (lack of normal development of intellectual capacities):

backwardness, slowness, subnormality, mental retardation

WordNet sense 3 (a decrease in rate of change):

deceleration, slowing

WordNet sense 4 (the act of slowing down or falling behind):

lag, slowdown