Wow that is some crazy growth. I had got my seeds from a bonsai tree kit and had grown them in San Diego for about 2 years before moving. The kit had some strange custom soil that I used for a while before moving it to a larger pot. I thought that moving it into a larger pot larger pot would have made it grow larger since it would have more room and I wanted my tree to be fairly large. However, I was told a few months ago that moving it to a smaller pot would cause it to grow larger since the roots would get crowded and then the growth would go to the trunk and branches (I am not sure if that makes sense but I made the move). Is there anything I should do to speed up growth, or have I missed the window for growth?
I'm not so sure about that advice, if you have a look at the number of old bonsai trees in pots they generally have very little trunk growth in 20 years in part because of the pot, but also because of bonsai techniques such as pruning and root trimming that gets applied, trees in pots are generally in refinement and the focus shifts to things like ramification and foliage size, but it also depends on the approach being used, some people grow there trees only in pots at a much slower pace as there are advantages in doing this.
It's winter for you currently, so i would avoid repotting until spring next year. But comparing growth rates for a bonsai is a tricky thing as there are number of things that come into play with growing trees larger, fertilizing, container size, sunlight, watering, techniques currently being applied and plant genetics. I mention these things as i have 6 jacarandas that i'm currently experimenting on, two where obviously genetically weaker and has never grown as fast as the others and will never be able to compete with the others speed of growth, 4 i did root work and 2 trunk chops to compare the results, root work substantially slows down the plant growth rate, the two with trunk chops are almost twice as thick as the other 4 now.
So if your are planning on repotting next spring and the aim is growth attempt to damage the roots as little as possible. As i mentioned, if you can put it in the ground, this will be first prize, if not grow it in a grow box, if this is not a possibility, grow it in a slightly larger pot. Gardening rule of thumb is about a inch larger pot and upgrading the pot size either every year or every other year depending on the trees growth rate and health until the tree reaches the required trunk thickness. Just be careful on the pot size, as too large a pot could result in the tree sitting in too much moisture for too long a period can get root rot and die. You will need to apply a fine balancing act, as you will need to work on root spread while growing the tree out, but also don't want to disturb the roots too much. The same applies to the foliage, you want it to grow out of hand a bit (which will help thicken the trunk) and then cut it back to a more manageable size (which will slow growth down again) constantly keeping an eye on branches that will be kept for the future tree and branches that are purely there to thicken the trunk