Improvement In Lumped Element Analysis?

I am modeling a twisted pair of wires in Simulink as a lumped element model and then pushing a square wave through and analyzing the output. Given the Lumped elements are frequency dependent, I run a sweep at a high frequency and then another sweep at a low frequency and aggregate and weight the outputs to create an accurate output wave.

It seems like there should be a way to pass your input square wave through a Laplace or Fournier transform and then run the calculations. Increasing accuracy of your simulation by calculating at all frequencies rather than just 2. Is there a better way to run this Sim that would both increase our accuracy and increase the speed of results? Maybe using Z-Transforms or S-parameters? Or passing the input/output wave through some sort of transfer function?