The present study had two aims. First, we set out to evaluate the structure of processing speed in children by comparing five alternative models: two conceptual models (a unitary model, a complexity model) and three methodological models (a stimulus material model, an output response model, and a timing modality model). Second, we then used the resulting models to predict multiple types of reading, a highly important developmental outcome, using other well-known predictors as covariates. Participants were 844 children enrolled in third through fifth grade in urban public elementary schools who received 16 measures of processing speed that varied in the above dimensions. A two-factor complexity model that differentiated between simple and complex processing speed was the preferred model and fit the data well. Both types of PS predicted reading fluency, and complex (but not simple) PS predicted single word reading and comprehension. Results offer insight to the structure of processing speed, its relation to closely related concepts (such as executive function), and provide nuance to the understanding of the way processing speed influences reading.