[Context] Microservices enable the decomposition of applications into small, independent, and connected services. The independence between services could positively affect a project’s velocity, which is considered an important maintenance metric measuring the time taken to implement features and fix bugs. However, no studies have investigated the causal relationship between microservices and velocity. [Objective and Method] The goal of this study is to investigate the effect of microservices on velocity which is a common maintenance metric. The study compares projects on GitHub developed with microservices style from the beginning and similar projects using monolithic architectures. The study was conducted as a retrospective cohort study,which is a study type used to assess causality. [Results] The results did not find statistically significant differences in mean velocities in microservice-based and monolithic projects. Furthermore, the statistical adjustment performed to quantify the statistical impact of the use of microservices on velocity considering additional confounders did not find statistically significant impact from these. [Conclusions] The results did not indicate a difference between microservices-based projects and monolithic projects in terms of velocity. In addition, this study will contribute to the body of knowledge of empirical methods and be among the first works to adopt the methodology of the cohort study.