It's meant to be simple and easy to use, and therefore it has been made as a custom editor of MeshFilter. It needs no preparations.
Drop the file in an Editor folder.
- store the normals length i editor prefs
- adjust the normals length in the mesh filter inspector
Thanks for pointing out the performance issues! One of the things I work with is performance optimizations, so it was rather embarrassing seeing this script again. I fully agree with you, and will update the gist. Thanks!