To add to the previous post, nurseries use progressive up-potting (1 to 2 to 3 gallon in successive seasons, for example) to promote growth.
If I were trying to speed this thing up, I'd chop it's height about in half, then in repotting season, check out the top of the roots and see if it may need the upper roots worked and substrate refreshed on top (in other words, a half repot of only the surface roots*) or if water isn't draining through a full repot. Hell, you could go for the chop and root prune as a way to maybe fast track it, as long as you're comfortable with the risk.
* there's a Japanese name for this procedure, but it's name currently escapes me