It is fairly intuitive that more complicated geometries, and geometries with many ports, will require more computational resources for solution than simpler geometries with fewer ports. A number of other factors influence resource usage in a simulation; some of these factors are summarized in the following list.
The use of anisotropic materials, as well as materials with nonzero electric and magnetic conductivity, require solution of a linearized eigensystem to obtain modes of wave ports. As a result, port solutions take longer to generate as compared to an identical system with only scalar materials and materials with zero conductivity.
A common mistake is to use a very dense set of frequencies, and this slows down the simulation substantially. Even if the frequency sweep is set to GAWE, so that a full solve is performed at only a subset of frequencies, each additional frequency in the full set adds to the simulation run time. In every case, the simulation will complete more quickly if a smaller set of frequencies is requested.
Wave ports typically result in a heavier computational burden than lumped ports, because the wave port solution refines the mesh before the volumetric solution is begun. Much of the time, this additional burden is not substantial enough to matter. An exception is when the geometry has many conductors on the wave port plane, or many coplanar ports on that plane. As described in “Wave Ports”, the wave port eigensolver will compute one more mode than there are conductors on the port plane. As the number of conductors increases, this computation becomes more difficult, and more subject to numerical error. As a result, the solver may jump between mode solutions from one AMR step to the next, slowing convergence and leading to fine meshes on ports, which can in turn slow the volumetric solve. Lumped ports avoid this issue completely by not requiring an eigensolution on the port plane, and as a result lumped ports are preferred in this situation.
A simulation domain that is large compared to the source wavelength will have an extremely fine mesh, as described in “Initial Meshing”. You should always check the initial number of elements in the mesh to ensure that you have not set up a system with an unnecessarily large number of mesh elements.
As described in “Materials and Attributes”, boundaries with PML applied to them are padded with auxiliary mesh elements filled with a lossy material. Thus, use of PML directly increases the size of the mesh, and therefore it increases the computational burden substantially. For this reason, PML is only recommended in simulations where the accuracy of the radiating boundary is of paramount importance.