Group differences in test scores are a key metric in education policy. Response time offers novel opportunities for understanding these differences, especially in low-stakes settings. Here, we describe how observed group differences in test accuracy can be attributed to group differences in latent response speed or group differences in latent capacity, where capacity is defined as expected accuracy for a given response speed. This article introduces a method for decomposing observed group differences in accuracy into these differences in speed versus differences in capacity. We first illustrate in simulation studies that this approach can reliably distinguish between group speed and capacity differences. We then use this approach to probe gender differences in science and reading fluency in PISA 2018 for 71 countries. In science, score differentials largely increase when males, who respond more rapidly, are the higher performing group and decrease when females, who respond more slowly, are the higher performing group. In reading fluency, score differentials decrease where females, who respond more rapidly, are the higher performing group. This method can be used to analyze group differences especially in low-stakes assessments where there are potential group differences in speed.