The velocity of light is a constant. The perception of it being slower when passing through a non-vacuum is due to the time lost to photons being absorbed and re-emitted by the matter it is passing through. During those times when the photon (or any massless particle) exists as an independent particle, it travels at exactly C. Refer to the field of quantum electrodynamics for details.
That is a change in velocity of light in the presence of a magnetic field.
Credit: Image: S. Giodini/A. Finoguenov/MPE