The Rasterization Discard override mode discards primitives immediately before the rasterization stage, but after the transform feedback stage.
This override mode is only available for OpenGL* ES 3.0 or higher and OpenGL 3.2 or higher (Core Profile).
If this override mode does not significantly impact frame time and GPU duration metric, then vertex processing consumes most of the GPU frame time and you should try optimizing the vertex shader(s) code.
A significant drop in frame time and GPU duration with this override enabled indicates a potential bottleneck in rasterization, fragment shader, or pixel processing stage. In this case, optimizing vertex shaders will not yield any significant performance gain. To identify the bottleneck, try other override modes, such as Simple Fragment Shader, Texture 2x2, or 1x1 Scissor Rect.