Question Terrain Updates and Navmesh

I am trying to work with Terrains and update the terrain at runtime but I also need the navmesh to update as well.

I have setup the terrain as a navmeshsurface so that I can call buildnavmesh on it when desired, but the building takes enough time to cause lag. I looked into multithreading but many of the functions when building need to be ran on the main thread so its not possible as far as I can tell.

If anyone has any suggestions I would appreciate it.