What is the speed of an electromagnetic wave?

In vacuum, they travel at the speed of light or 299,792,458 m/s

In air, they travel slightly slower at 2.997e8 m/s

In electromagnetism, permittivity is the measure of how much resistance is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. Permittivity is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material. Thus, permittivity relates to a material's ability to transmit (or "permit") an electric field.

In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter ?. The term was coined in September, 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity.

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The speed of any EM wave in free space is the speed of light.
c = 1/sqrt(mu0*epsilon0) = exactly by modern definition of the meter = 299792458 m/s


Optically dense media have different effective speeds of EM waves because of enhanced permittivity and/or enhanced permeability, as per the formula:
v = 1/sqrt(mu*epsilon)