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.