Hfss includes both a 3D and a layout driven interface. In the layout , you can solve with either 2.5DMom of FEM. You can compare for the same design. 2.5D can be a bit faster if you neglect thickness but the Fem solution is faster when you consider 3D ( metal thickness or via radius). Also Fem has a better adaptative refinement which give you good confidence in the result. Because the full Fem solution with adaptative is provided in a few minutes on common design without any approximation, this should be preferred in most situation.