One obvious problem is that `mr.use_simplify_normals` was assigned after
face corner normals were retrieved. The other more complex problem is
that now the normals caches automatically mix custom normals from other
domains. This can cause the expensive "Tangent Space" normals to be
calculated even though we don't explicitly request face corner normals.
To fix this, clarify the purpose of the option to only apply to that custom
normals format and use the true normals instead in that case.
Pull Request: https://projects.blender.org/blender/blender/pulls/140879
2e568d31ed