ID 367590 - Slow viewport performance when vertex normals are displayed

Problem summary
Slow viewport performance when vertex normals are displayed. 

Customer reported version
Modo 12.1v2

Customer reported platform
Windows 10 

Steps to reproduce

1) Open a new scene in Modo
 

2) Create a cube and select all polygons. 
 

3) With polygons selected, subdivide the cube six times by pressing Shift+D or go to Geometry->Polygons->Subdivide. 
 

4) Go to Vertex Map->Create. Select "Vertex Normal Map" as the map type.
 

5) Go to View->"Show Vertex Normals" to enable vertex normals.
 

6) Press F5 to open Command History. Enter "glmeter" in the Command field. The frame rate should now be displayed in the viewport.
 

7) Rotate in the viewport and observe the low frame rate. 

Workaround

1) Go to View -> "Show Vertex Normals". Open its properties. 

2) Disable "Cull Hidden Normals". 

3) Rotate in the viewport. The performance should now increase. 

Reproduced by support

This bug has been reproduced in:
Modo 12.1v2 - Windows 10 - MacOSX 10.13 
Modo 12.0v1 - Windows 10 
Modo 11.2v3 - Windows 10 
Modo 11.1v1 - Windows 10
Modo 11.0v3 - Windows 10
Modo 10.2v4 - Windows 10 - MacOSX 10.13 

Earliest version tested
Modo 10.2v4 - This issue appears to occur in all versions on Windows 10 and MacOSX 10.13 

Expected behaviour
Performance should not drop in the viewport when "Cull Hidden Normals" is enabled. 

Actual behaviour
Performance drops even though "Cull Hidden Normals" is enabled.