Studying seismic wave propagation and evaluating the dynamic response of structures and infrastructures poses several challenges. The Domain Reduction Method (DRM), originally proposed by Bielak et al., represents an effective strategy for reducing computational complexity while maintaining high accuracy in the region of interest. The methodology is based on splitting the problem into two sequential steps: a free-field simulation is first performed, and then used to compute equivalent forces applied to a reduced domain. In this work, we present an analysis of the DRM and its parallel implementation within the open-source Fortran code SPEED (Spectral Elements in Elastodynamics with Discontinuous Galerkin) developed by Mazzieri et al., which is based on the Spectral Element Method (SEM) for simulating seismic wave propagation in heterogeneous viscoelastic media. Since the DRM involves splitting both the domain and the simulation phases into different parts, independent meshes, partitioned in a balanced manner, are considered to improve generality and efficiency. The parallel implementation within SPEED is validated through three-dimensional numerical tests under various scenarios, ranging from plane waves, both vertical and inclined with arbitrary angles, in homogeneous media, where analytical solution is available, to heterogeneous media and domains featuring more complex and realistic topographies. Finally, the coupling of the soil model with structures is addressed to study soil-structure interaction, with particular attention to strategic structures in the energy sector.