We probably do need dedicated documentation on this, but the basic principle is simple enough: if you have 3 sprites stacked on top of each other, then each pixel is drawn to 3 times, because C2 renders back-to-front. That's overdraw (filling pixels more than once), and it wastes fillrate. Using a force-own-texture layer (which includes options like opacity which implicitly turn it on) causes the entire layer to be overdrawn since it has to draw the layer's own texture to the screen.