low shader quality hurts FPS

So I was playing fh4 and decided to lower the graphics option one by one to see how the game would change, at max graphics option I was getting 200 fps with 60 to 80 gpu usage

I started lowering the options and everything was fine, the fps started going up like crazy, but when I got to the last option, Shader quality, I set that to lowest, restarted the game and the fps went to a max of 30, gpu usage of 90 to 99, I set them to max and restarted the game everything was working like it did before

anyone knows why?


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