Transistor switching time is limited mostly by electrical capacity which is determined by the permittivity of the material the transistor is made from. And perm

Transistor switching time is limited mostly by electrical capacity which is determined by the permittivity of the material the transistor is made from. And permittivity and permeablity are the only things determining the speed of light in a material. So saying that speed of light is the limiting factor of transistor switching time is valid.