It does use the bounds.
However a layered grid graph sometimes needs to create new node layers if you add geometry on top of other existing geometry. This can cause large spikes in the calculation time since it needs to go through all nodes. Also, the update might need to recalculate all areas (islands of nodes not connected to each other), this can also cause a large spike (but should do so for grid graphs as well). In certain situations when you know that your update will never separate two areas of the graph, you can pass requiresFloodfill = false to the GraphUpdateObject ( see http://arongranberg.com/astar/docs_FeatureFreeze/class_pathfinding_1_1_graph_update_object.php#a2efbcd7b73c47dce7781034f38b35da6 ).