The world often goes too fast for children with autism spectrum disorder (ASD) to process. We tested the therapeutic effectiveness of input slowing in children with ASD. Over 12 months, 12 children with ASD had weekly speech therapy sessions where stimuli were slowly played on a PC, while 11 age- and level-matched children with ASD had speech therapy using real-time stimuli. At the beginning and end of the study, all participants were assessed on communication, imitation, facial emotion recognition, behavior, and face exploration. Whereas communication and facial emotion recognition improved in both groups, imitation increased, inappropriate behaviors decreased, and time spent fixating mouth and eyes increased solely in the group using slowness. Slowness therapy seems very promising for ASD children.