Guilin Sun
Ansys Employee

If you are not simulating SPP effect, I would suggest you to use PEC, or better 2D sheet PEC:

https://optics.ansys.com/hc/en-us/articles/360034915113-Standard-optical-conductivity-material-models-in-FDTD-and-MODE

Bulk metals have large permittivity with heavy loss. FDTD needs very fine mesh to resolve the physics. Therefore it needs a lot meshes so huge memory requirement.