I'd like to mention that the EI interface should tell the editor to wait processing navmeshes until all supplied terrain meshes has been created, I am noticing on areas with res 128 and heightmap density 128, it waits a long time to process the terrain as it seems the editor keeps sending updates per area to change the navmesh, this causes a long stall long for the editor to finish each area, longer than it needs to be.

PS: physics can also wait until the render terrain mesh has been set, possibly by letting the EI have a lock-unlock call to these.