Good point about longer scions. I have seen that. I ended up finding how Luther Burbank sped this up:
http://www.lutherburbankonline.com/V1-C6.html
And now, with our sprouted cherry seedlings six inches or eight in height… we come to the most important short-cut of all-quick fruiting, so that there may be quick selection…
… Graft away out at the tip ends of the tree," thought Luther Burbank, “and you will save from two to seven years of time.”…
… if we graft close to the trunk we can, at best, attach but six, or eight, or a dozen scions. But if we graft out at the tip ends, we can put five hundred scions on a single tree."*
*Grafting was nothing new; but it remained for Luther Burbank to learn the secret of producing, by means of it, five hundred different kinds of fruit on a single tree at the same time, so that a comparative test might be made. It remained for Luther Burbank, with his theory of starving a living thing to make it hasten its reproduction, to cut from two to seven years out of the long wait for the fruit which is to tell the story of the heredities which were confined within the seed.
…while the average time of fruiting has been brought down to less than two seasons, in some exceptional cases Mr. Burbank has secured fruit for testing the same season that the graft was made…
the method is to save time for the individual plant as well as to provide short-cuts for the process of evolution. To go back to our cherry seedling, now six inches above the ground, if we were to depend on nature’s processes, by careful planting and cultivation we might produce cherries in seven years; but by short-cutting through grafting, and short-cutting grafting itself through Mr. Burbank’s plan, we shall have our cherry crosses in 1914 instead of in 1920-five hundred of them all on a single tree…