The transition from sticking to sliding of a frictional interface does not happen uniformly. The shear resistance of the interface is usually reached at a single point from which an interface rupture starts to propagate (Rubinstein et al., 2004). The repetition of such local slip events results in a global stick-slip behaviour, or transitions into global steady state sliding. However, the physics of these local slip events is highly complex and not well understood. It is observed that the propagation speed of the slip front, which is of great importance for earthquake sciences, is varying along its path (Ben-David et al., 2010). Using the finite element method, we simulate the propagation of slip fronts at frictional interfaces and analyse the rupture speed. We show that the higher the needed relative rise of the energy density at the slip tip is, the slower the rupture propagates. This leads to an energetic criterion to describe the speed of interface rupture (Kammer et al., 2012) and underlines its similarities with classical fracture mechanics.