Discussion on the slower simulation speed of EM FDTD for lower frequency electromagnetic field

Hello everyone, I am currently doing some simulations of electromagnetic fields on the human body. In the process of using the EM FDTD module, I found that the module is not suitable for the analysis of low-frequency electromagnetic fields during the process of viewing the instruction manual and the actual simulation. You can use ARMA for analysis, but I don't know how to use it. I hope you can give me some suggestions.

what is ARMA?

FDTD is not adequate for any simulations where the wavelength is long compared to the object being modeled