You're always driving a waveguide that has inductance and capacitance per unit length. But when resistance is high enough you are just charging a big distribute

You're always driving a waveguide that has inductance and capacitance per unit length. But when resistance is high enough you are just charging a big distributed capacitor. It's a different regime from the high-conductivity situation (which looks like a transmission line). Either picture could apply at sub-ns speeds, it depends mainly on the resistance.