This article is evaluating parallel processing in electromagnetic simulations for computationally demanding application, using developed FDTD code. A waveguide lens antenna which is complex and large in terms of wavelengths has been simulated and a prototype has been fabricated and measured. Simulated and measured radiation patterns are compared and showing good agreement. The approach of using parallel version of QuickWave-3D software on a popular multi-CPU shared-memory computer has been found suitable for demanding simulations of complex structures.