The CPU in modern graphical pipelines is only really doing a couple of things. Predominantly handling the drawcalls, gamelogic (player interactions, phsyics), and audio.
Where you will get the most reduction in CPU load is by lowering world geometry. Meaning object quality and draw distance. Shadows, depending on the type, can also have an impact because it requires geometry to know where the shadows land. If it's done on a GPU shader less so.