Rather a lot i would say, the speed of the lightwaves is c/n with n the refractive index, and here are some: List of refractive indices - Wikipedia, the free encyclopedia
Light in silicium goes as slow as one-fourth speed of light!

I would also like to add that you do not really need quantum mechanics. From the electromechanical point of view the fields do contain energy, they are the medium. As such, the physical laws of the electrodynamics are Lorentz invariant; that is, they are still valid whatever your speed is.

In special relativity the transformation of positions is different then what we classically do, which is the Galilean transformation, not the Lorentz one! The mistake with the ether of at the beginning of the previous century and before was that people did not realize that the Lorentz transformation was to be used, and that the Galilean one is not consistent.

Ill leave the question for someone else

BTW eh not up to the wave-particle duality discussion, should follow a philosophy of QM course