[Context] Microservices enable the decomposition of applications into small and independent services connected together. The independence between services could positively affect the development velocity of a project, which is considered an important metric measuring the time taken to implement features and fix bugs. However, no studies have investigated the connection between microservices and development velocity. [Objective and Method] The goal of this study plan is to investigate the effect microservices have on development velocity. The study compares GitHub projects adopting microservices from the beginning and similar projects using monolithic architectures. We designed this study using a cohort study method, to enable obtaining a high level of evidence. [Results] The result of this work enables the confirmation of the effective improvement of the development velocity of microservices. Moreover, this study will contribute to the body of knowledge of empirical methods being among the first works adopting the cohort study methodology.