Computerized reaction time tasks on working memory, selective- and sustained attention were collected in twins at age 5 years (N = 474 children) and at age 12 (N = 346 children). The longitudinal correlations of processing speed on all tasks were substantial (0.38). For slope (i.e., the delay caused by higher memory load) and fluctuation in tempo the longitudinal correlations were 0.08 and 0.26, respectively. The results hinted at genetic factors being an important mediator of stability of executive functioning over time. Also, genetic variation was the most important explanation for individual differences in executive functioning at both ages.