unskillfulness - a lack of cognitive skill

pace, rate - the relative speed of progress or change; "he lived at a fast pace"; "he works at a great rate"; "the pace of events accelerated"

leisureliness - slowness by virtue of being leisurely

dilatoriness, procrastination - slowness as a consequence of not getting around to it

stupidity - a poor ability to understand or to profit from experience

mental defectiveness, abnormality - retardation sufficient to fall outside the normal range of intelligence

mental deficiency, moronity - mild mental retardation

amentia, idiocy - extreme mental retardation

imbecility - retardation more severe than a moron but not as severe as an idiot

Down syndrome, Down's syndrome, mongolianism, mongolism, trisomy 21 - a congenital disorder caused by having an extra 21st chromosome; results in a flat face and short stature and mental retardation