For complex scenes, many different approximation nodes with unique settings may be needed to ensure accurate and efficient renderings.
When previewing such scenes, you can temporarily lower the tessellation quality to produce low-quality renderings more quickly with global (scene-wide) overrides.
To override approximation settings
The Render Settings window opens. For more information, see Render Settings window.
In the mental ray tabs of the Render Settings window, the Options tab, Overrides section, Tessellation sub-section provides controls for specifying surface and displacement approximations to use for rendering. When specified, these settings override all approximation assignments in the scene. Selecting None will cause mental ray for Maya to once more respect the approximation assignments in the scene.