The velocity that electricity propagates is related to the dielectric in the cable. For EM waves in free space, the speed is that of light but there is only the base dielectric of free space: -
So as the permittivity (\$\epsilon\$) increases so velocity reduces.
The characterisitic impedance of free space is also related to those terms: -
Basically, if you compared both equations, as characteristic impedance drops (due to permittivity increasing), velocity of propagation also drops.
The product of speed and impedance is \$\dfrac{1}{\epsilon}\$
And the ratio of impedance to speed is \$\mu_0\$.
I've used \$\mu_0\$ above (rather than \$\mu\$) because there is no significant increase in magnetic permeability due to a cable's construction.