Recently introduced 3D NavMesh really needs Saving and Loading feature, as well as the “Generate NavMesh” action that could be called manually, instead of automatically at the scene startup or any level geometry change; adding this would eliminate the noticable freeze that occurs even when generating navmeshes for a fairly medium-sized scenes.
I recently experimented with the 2D Navmesh Extension and saving/loading it from a variable. Generating the navmesh itself for an extra-large scene (roughly 480 000x480 000px square with a small cell size, random obstacles all over etc.) took about 10-15 minutes, whereas saving, loading, and any subsequent use were instantaneous. I hope something similar is possible for 3D NavMesh as well.