Modularity is a desirable property of complex software systems, since it simplifies code reuse, verification, maintenance, etc. However, the use of loosely coupled modules introduces a performance overhead. This overhead is often considered negligible, but this is not always the case. This paper aims at casting some light on the cost, in terms of performance, that is incurred when designing a relevant group communication protocol with modularity in mind: atomic broadcast. We conduct our experiments using two versions of atomic broadcast: a modular version and a monolithic one. We then measure the performance of both implementations under different system loads. Our results show that the overhead introduced by modularity is strongly related to the level of stress to which the system is subjected, and in the worst cases, reaches approximately 50%.