This paper I was reading said: "At a high level, the propagation speed of EM waves through conductive materials is SLOWER than their speed in air".
Don't conductive materials intrinsically have less impedance than air as propagation mediums, to be conductive in the first place. Wouldn't the ray flow through this conductor faster with less 'resistance'?
Do EM waves only travel at the speed of light in a vacuum?
Or am I Stupid??
You need to go much deeper into the physics in order to understand what is really going on. The equation of the wave inside a conductor can be derived from Maxwell’s equations
$$\mathbf{E} = \mathbf{E}_0 e^{j(\alpha z -\omega t)} e^{-\beta z},$$
where \$z\$ is the direction perpendicular to the conductor in our case and
$$\alpha = \omega \sqrt{\mu\varepsilon} \sqrt{\frac{1}{2} + \frac{1}{2} \sqrt{1 + \frac{\sigma^2}{\omega^2\varepsilon^2}}}.$$
Here the first exponential is the wave and the second exponential is the attenuating factor. It can be shown that for a bad conductor, satisfying \$\sigma \ll \omega \varepsilon\$, the wave number is
$$\alpha \approx \omega \sqrt{\mu \varepsilon}.$$
It can also be shown that for a good conductor, satisfying \$\sigma \gg \omega \varepsilon\$, the wave number is
$$\alpha \approx \sqrt{\frac{\omega \mu \sigma}{2}} = \sqrt{\mu\varepsilon} \sqrt{\frac{\omega \sigma}{2\varepsilon}}.$$
Hence the speed of propagation for a bad conductor is
$$v_\mathrm{p} = \frac{\omega}{\alpha} \approx \frac{1}{\sqrt{\mu\varepsilon}}.$$
This is the same as the speed of light in non-conductive media. However in a good conductor
$$v_\mathrm{p} = \frac{\omega}{\alpha} \approx \frac{1}{\sqrt{\mu\varepsilon}} \sqrt{\frac{2\omega\varepsilon}{\sigma}}.$$
It can be seen that the speed of light you would expect to see is multiplied by a second factor, which is less than one due the fact \$\sigma \gg \omega \varepsilon\$. Thus we arrive at the counter intuitive result that the better the conductor the lower the propagation speed.
But wait, there’s more! We use conductors only as anchors for the wave. The wave propagates in the space around the conductor and only penetrates up to the skin depth. Note that \$z\$ is in the direction perpendicular to the conductor. Usually what we really care about is the propagation in the direction parallel to the conductor.
Not at all a stupid question. It looks like you’re reading a confusing paper.
Electromagnetic waves don’t generally flow very far through conductors. Light reflects from metal, it doesn’t go through it. Radio doesn’t propagate in sea water. These things happen because the mobile charges in the conductor, well, move and that cancels the wave as it penetrates in.
Glass and other non-conductors have charges that can move, but not freely. The motion of their changes acts to slow light down, but not completely cancel it.
With no material, light moves at the speed of light in a vacuum because it is in a vacuum.
