Yes. The overall density does indeed limit the speed of visible light depending on that very density of each medium measured. Water vs. air for instance. This also manifests itself in the other areas of the electromagnetic spectrum such as electricity in that it flows at different rates (speeds) thru different materials such as copper vs. steel.

While not a direct measurement of speed itself, the effect manifests itself in conductivity where copper has a great amount compared to substances such as ethylene plastics which are such poor conductors, they are actually used as insulators. It's not exactly the same thing that's being measured, but one can use the analogy as indicative.