Why does a differential signal travel faster than a common signal on a microstrip?
The primary quality that determines the speed of the differential or common signal is the effective dielectric constant the signal sees. In a tightly coupled differential microstrip, the differential signal propagates in the odd mode with electric fields that extend into the air above the signal lines. The field lines see an effective dielectric constant that is significantly lower than the bulk value due to the contribution of all that air. This lower effective dielectric constant means a faster speed and a shorter time delay. A common signal on the other hand, propagating in the even mode of the microstrip, has very little field contribution in the air above the trace. Most of the electric field is in the bulk material under the signal lines. This means the effective dielectric constant the common signal sees is closer to the bulk value. A higher effective dielectric constant means a slower speed and a longer time delay. If the lines in the differential pair are very weakly coupled,
