Material Quality Level
Materials can use the Quality Switch Material Expression node to disable some expensive Material parts that have a minor effect on the final look. To see the effect of this, you need to toggle to the Low Quality mode.
Whatever the Material Quality Level is set to, low or high, will determine which expressions get evaluated for this material (the low or high pin). The default pin will fill in either high or low (or both) if they have no inputs. This material contains 2 reasonably high-cost Perlin noise operations when set to high:
This example is to simply show off the Material Switch Node. The Noise nodes are really expensive making them really good for this example, but they should be used sparingly as there are cheaper ways to get this effect.
Material Quality Level -> Low
Material Quality Level -> High
Above, Shader Complexity mode shows us that our high quality material is more costly than other shaders, with darker greens meaning more costly shaders.
Using the quality switch causes more shaders to be compiled (shader permutation).
This feature is not for distance LOD as you cannot have two quality levels at the same time. The feature can be used to reduce:
Shader computations (e.g. disable fuzz layer).
Texture lookups (e.g. no detail bump map).
Memory bandwidth (e.g. using fewer textures).
Most material editor outputs only affect the pixel shader. World position offset and all tessellation outputs affect the other shader types. Pixels shaders only cost a lot if they occupy large portions of the screen (skybox for example), while other shaders only matter if the object is not culled (inside the view, not hidden behind opaque objects).