Can someone please give me some general comments or insights to the simulation time of mechanics and diffusion (assembly time only, i.e., computation time other than solving the linear equations)?

Same mesh is used.  Finite element method with linear elements (triangle or tetrahedra) is used for elastic deformation.  Finite difference method with trapezoidal rule and backward difference is used for diffusion of a single dopant and assembly is done node-wise.  Thanks!