The hole was much smaller before. It's getting bigger & more bark areas around it are cracking now. The hole & around areas are dry to the touch. Tree is 9 or 10 years old. It grows slower for it's much smaller than other trees, which are different types - a cherry and a crabapple, all similar size when planted years ago together.
Anyone has any idea about the causes, effect, and any solution that can stop it from getting worse?
It looks like apple canker. See here: https://www.rhs.org.uk/advice/profile?pid=63
If you have an attack as bad as your picture on the trunk of the tree, there is not much you can do. The only "cure" is to cut out all the affected wood but you can't cut away half the trunk, for obvious reasons.
If you have other apple trees, consider getting rid of that one to avoid cross infection. It will probably reduce the apple crop and eventually kill the tree in any case. As you said it is already slowing the growth rate, in the same way that stripping a ring of bark from half the circumference of the trunk would do.
I suppose you could completely remove the trunk below the canker and continue to grow the two side branches below it, but the shape of the tree would look a bit weird!
