Signal speed is roughly inversely proportional to the square root of the dielectric constant.
A low dielectric constant will result in a high signal propagation speed and a high dielectric constant will result in a much slower signal propagation speed. A good analogy is to imagine yourself running along the beach with your feet six inches deep in the water. The dielectric constant is analogous to the viscosity of the water. Dielectric constant is not an easy property to measure or to specify, because it depends not only on the intrinsic properties of the material itself, but also on the test method, the test frequency and the conditioning of samples before and during the test. Dielectric constant tends to shift with temperature.