From what I understand, to reduce the time taken for public key generation, you can use nested trees in XMSS to form Multi-tree XMSS.
For an instantion of 2 layers, it seems that the leaf of each L tree (one of these nested trees) is an OTS public key. This means that the leaves of the top level tree sign the OTS keys in the L tree below it.
I read that this allows you to only compute the top main tree and only the first tree in each layer, which naturally saves a lot of storage and computation time.
However, I do not understand how this is possible because to compute the top layer tree, you need to know the root of each L tree, which involves computing the whole L tree below each leaf.
What am I missing here?