Hi guys,
The time domain has an inherent limitation. In order to reduce the time step it needs to refine the grid (and the other way around).
I recommend to test Ansoft HFSS which uses Finite Element Analysis. Not only that it has no refinment problems, but also you can control the accuracy of the convergence process. In practice you can choose if you want a "fast" solution (by specifying 5% accuracy or any other number ) or wait a lttle longer until it converges (to 2% accuracy). In addition you can control how much adaptation it does in each pass (refinment per pass) - to accelerate the convergence.
Regards,
Itai