Every heart shot is not the same. If you open the heart, cut atria or ventricles open so that they leak, cut the major veins or arteries leading to and from, the animal will die quickly. If you cut the heart muscle but don't do the above, it will die less quickly (from that wound), however, remember, to get to the heart you must put a hole in the chest cavity and likely one or more lung as well.
Also remember, , a deer on speed can run 100 yards in seconds. So a deer so hit may walk 20 yards and collapse and that same deer, running like hell may very well go 120 yards.
ChuckC