The mechanism of energy dissipation near a fast running crack tip is studied. The presence of low-sound velocities in the plastic region should imply increased resistance to the energy flow from the elastic strain field at increasing crack-tip velocity. Numerical calculations indicate, however, that this is a rather weak effect in cases of small-scale yielding. On the other hand, increased resistance to ordinary plastic flow at increasing crack-tip velocity is found to be a strong effect. Increased resistance to plastic flow implies a smaller plastic region and thereby a smaller energy dissipation in this region. In the region where material separations are initiated, very different stress fields prevail at fast and slowly growing cracks. The implication on the morphology of material separation suggests that the associated energy dissipation should be much higher for very fast growing cracks than for slowly moving ones.