I can't speak for all games, but I find my fragment shaders dominate. That is compute power related directly to the number of pixels rather than polygons.
If I can render objects in the distance at 1/3 the spatial resolution, that is 1/9th the fragment shading, times a much reduced texture bandwidth since I'll be using a lower resolution mipmap.
I'll still have to composite it back in to the full resolution frame, but if that operation is faster than my fragment computations it would win.
That side scroller benefited because it could just draw the background/foreground sprites at a lower resolution before blurring. You wouldn't be able to get that luxury easily with a 3D game.