Modularity plays a crucial role in today’s manufacturing systems by offering them better flexibility, adaptability, efficiency and other positive effects. However, achieving the optimal modularity requires aligning modularity aspects with the performance goals of the manufacturing system. In this article, it is aimed to simultaneously minimize manufacturing lead time (MLT) and maximize the overall efficiency of manufacturing assembly processes. In this context, this paper explores the impact of theoretically determined “optimal” manufacturing process modularity on the two selected performance indicators, namely MLT and process efficiency of assembly line structures. For this purpose, selected optimal modularity indicators will be tested to evaluate modularity influence on system performance parameters trough a practical case study. The research results offer practical guidance for manufacturing practitioners, how modular design can drive improvements in the manufacturing performance objectives. Moreover, this article contributes to the theoretical and practical knowledge on manufacturing modularity and offers the methodological framework for its effective implementation in real-world industrial environment.