Insulation does affect the EM propagation speed along the outside of a wire
This becomes very apparent with transmission lines ( antennas) that have either a covering
The result is a slowing of the EM propagation, This is known as the VF -- Velocity Factor
That is what i'm asking.If it does,why a magnet interacts it?i'm confused.please explain it??
Why do you think a magnetic field interacts with insulation? It does not. As stated already, insulation has a higher dielectric constant than air, so it will affect the E field generated by voltages on the wire (including the velocity factor that Dave has mentioned).
I guess if you're counting a sheath (like in coaxial cable) as the insulation, then it certainly affects the [itex]B[/itex] field produced by the wire because the metal intentionally has high permeability. Otherwise plastic wouldn't have a noticeable effect.