There are a number of subtleties to this. When considering electromagnetic waves, it is a mistake to focus on the properties of the conductor. What controls the speed of the wave is the nature of the dielectric medium that the wave travels in, if any. Electromagnetic waves do not require any medium. They travel freely in space or other vacuum.
In coaxial cable, the dielectric is the insulation between center cable and shield. In a parallel transmission line (e.g., twinlead cable), the dielectric is air and whatever material is in between the conductors. On a circuit board, the dielectric is a mixture of air and the circuit board material for traces on the outer layers (top and bottom). For traces buried between ground and power planes, the dielectric is the circuit board material alone.
In vacuum or space, there is no dielectric. In air, air is the dielectric.
In waveguides, air is the dielectric, unless the waveguide is evacuated which is sometimes done for high power signals. But in waveguides, the electromagnetic wave effectively bounces back and forth down the waveguide. So even though the dielectric is air, the signal propagates more slowly than a signal radiating normally through air.
For air, vacuum, or space, you can take the speed to be the speed of light, roughly 299 million meters per second. In a coaxial cable it really depends on the type of insulation. Often it is around 60 to 80 percent of the well-known speed of light. In circuit boards, it is around 150 million meters per second for buried traces, and a little faster for surface traces.
Practically speaking, the speed of light is determined by the relative dielectric constant of the medium. This dielectric constant is not necessarily the same for all wavelengths. In other words, when radiation passes through a dielectric, it may be distorted in such a way that certain wavelengths are delayed more or less than others.
You can read about the "velocity factor" here: