Pathfinding in general doesn't allow a lot of multi-threading. You can do bi-directional search, but it doesn't always work as good as it seems on a 2D maze[0], and adding workers beyond the 2nd requires being really good at guessing mid-points, but it's doable in domains with sub-goals.

[^0]: Not the case here, but some domains branch too much when going backwards and can easily explore states unreachable from the start.