Less graphics for more CPU = more FPS?

[Willysonicteam]

Good afternoon, have a doubt.
What is the reason? deactivating the "alpha PPase" in Mario Galaxy 2, won many FPS in speed (5 or 6 depends zones), but, deactivating " the lighting" and "fog" have the same FPS?
I have a Gforce 8800GT, graphics power problem no is.
Is the cpu ¿no?. Work is less?
Could be implemented in future revisions more options to "deactivated"so that the CPU work is less? and have more FPS.
thanks.

[DacoTaco]

ive never bothered to look into the details but it is a fact that on some systems decreasing the amount of shown textures increases game speed

ive always guessed this was because by decreasing the amount of processing work of the gpu thread; the processing time could be given to another thread to speed it up. (for example, phantasy star online ep1&2 has a nice speedboost on my system when i enable my no texture AR hack since it disables a shit ton of textures )
dont hold my word for that theory tho ; i never bothered looking into it and im only using logic guessing here