Nursing naturally increases oxytocin levels, and so can speed labor once it starts. Nipple stimulation is often suggested as a natural alternative to pitocin, and so this is just another option for a mom who is nursing. Nursing will also speed delivery of the placenta.
Some mothers nurse during early labor, but usually because the child needs it rather than as a means to speed labor. Many mothers feel that nursing during active labor would be too distracting, but this (like labor) will vary from mother to mother.