Actually, the speed of moss wall growing is related to the way you tie your mosses to the wall also. There are 1001 ways to do it. But which is the best?
To me personally, the one that causes the least damage to the moss would be the best and fastest.
*Disclaimer: Context of having same water conditions/environment. Fastest in term of the growth speed of same moss sp. Bearing in mind that different moss grows in different speeds.