Honestly this seems to be something that's common in a lot of games released in recent years. I turn a setting down and may or may not see a 1-2FPS difference and even when I do notice it I don't really know if it's because of the change in setting or just something else like poping out of the menu.I think my main problem I have with the performance is that it barely scales at all with the different settings for me.
If I turn everything down to low, I still only get 80-100fps at 1440p with a 1080.