Well, I optimized it more
Basically what I did is that the tiles of the terrain hierarchy that are visible in the moment when the crater is created are updated first. Tiles that are affected by the crater, but are only going to be visible when you move closer or further from the crater, are updated on demand.
For larger craters it may take longer in some circumstances, like when you are quite close to them. During game play it will hardly matter though, since it will likely blow you away anyway, and even if it didn't the occlusion by dust and smoke will hide the crater for quite a long time.