You can join more Visuals3D in one . It's fasted if you reduced the Visuals3D count.

Example:

MeshBuilder.Append ( yourMesh )

If you do not need to manipulate the separeted models , you can join them.

Hi, I think that you forgot BackMaterial Property

Add this line in your code

GeometryModel3D.BackMaterial = MaterialHelper.CreateMaterial(brush)