Aims: Although IDS is typically described as slower than adult-directed speech (ADS), potential impacts of slower speech on language development have not been examined. We explored whether IDS speech rates in 42 mother–infant dyads at four time periods predicted children's language outcomes at two years. Method: We correlated IDS speech rate with child language outcomes at two years, and contrasted outcomes in dyads displaying high/low rate profiles. Outcomes: Slower IDS rate at 7 months significantly correlated with vocabulary knowledge at two years. Slowed IDS may benefit child language learning even before children first speak.