[ELI5] Why does Depth of Field (DoF) setting in video games usually take a toll in performance?

Question

By blurring the background. Your basically lessening the need to render and reduce resolutions. So it would logically make sense for the increase of the games FPS. However it seems it turning on does oddly the opposite? And I wanna know why that is

In: 48

Answer 1

Wow, so basically the game has to do math? No wonder it’s slowing down!