When spawning on very large terrains in particular Tree spawning can become very slow.
The reason for this is that Gaia adds a collider for every tree, and then parents these all the the same parent. The reason for the collider is so that Gaia does not spawn two trees in the same location as terrain tree colliders do not work in edit mode.
At the time of development parenting seemed like a great idea, however it turns out that every time you add another child, every child recalculates its position, and this can get exponentially slow. This was solved in GeNa, and the solution will be rolled into version 2 of Gaia.
In the mean time there is a nice work around. Click on something outside of the unity editor... wait a little.. then click back into the unity editor and it will be done. Because unity wont update while not selected, this nicely avoids the recalculation issue.