Individual differences in language processing speed have been argued to play an important role in the acquisition of new words. Inconsistencies across studies, however, mean that it is not clear whether this relationship is causal or correlational, is present right across development, or whether this relationship extends beyond word learning to affect other aspects of language learning, like syntax acquisition. To address these issues, the current study used the looking-while-listening paradigm devised by Fernald et al. (2001) to test the speed with which a large longitudinal cohort of children (the Language 0-5 Project) processed language at 19, 25, and 31 months of age, and took multiple measures of vocabulary (UK-CDI, Lincoln CDI, CDI-III) and syntax (Lincoln CDI) between 8 and 37 months of age. We found that processing speed correlated with vocabulary size - though this relationship changed over time, and was observed only when there was variation in how well items used in the looking-while-listening task were known. Fast processing speed accelerated subsequent vocabulary growth, but only for children with smaller vocabularies. Faster processing speed did, however, lead to faster syntactic growth, even when controlling for concurrent vocabulary. Thus, this work is the first to show a causal relationship between processing speed and syntax acquisition. The results indicate a relatively direct relationship between processing speed and syntactic development, but point to a more complex interaction between processing speed, vocabulary size and subsequent vocabulary growth.