Grid graphs (which you use) cannot be calculated in a separate thread because they use the unity physics engine for a lot of things, and that can unfortunately not be accessed from a separate thread. It is possible that the new unity job scheduler will be able to improve on this, but currently it is not possible.
You can however scan the graph over several frames using the ScanAsync method (which has superseded the ScanLoop function). It does not guarantee a good frame rate though, but it will be better.
Another approach you could take is to issue many smaller graph updates to recalculate only a part of the graph for every frame (for this I would strongly recommend to use the latest beta though, due to some optimizations done in that when making small updates to large graphs).