Explain Conductor-Dielectric interface HF interference

I’d like a fairly understandable explanation for the high-frequency interference that is observed at the very interface where a conductor meets a dielectric (a plastic one, let’s say)
I understand this to be one of the primary issues that make highly porous (much nitrogen/air per volume) dielectrics a superior choice, or even nitrogen/air tubes. Apparently it even helps to have the polymer just loosely hanging from the conductor.

Is this phenomenon entirely separate from the smearing of the signal by arrival time discrepancies resulting from differing speeds at the conductor surface, the dielectric and air?