Conversion to World Partition very slowly

Hello, I am trying to covert a large map to world partition.
My UE is 5.1.0. The map is the demo map (/Content/ModularVictorianCity/WC_Maps/ModularVictorianCity.umap) of “Next Gen Modular Victorian Neoclassical City (Bundle - Exterior & Interior)”, which can be downloaded from their product page. I tried some other big maps, and get similar results.
When I use the tool in editor to convert it, the editor stay 0% for over two hours. The CPU usage is about 15% (it is Intel i7-9700k), so the editor is doing something; it uses a large amount of memory; and its disk i/o is nearly zero.

The UE Editor seems cannot use all computing resources when handle complicated Blueprints. I think it stuck in some single-thread tasks, and some algorithm about Blueprint has a unoptimized time complexity. I met similar phenomenon before when handling complicated Blueprints. For example, changing about one thousand static mesh components of one actor from moveable to static.

Im having same issue in 55.1, is there a fix ?