Increased performance – Mipmapping increases cache efficiency as full-size textures will not be needed as often, and the lower resolution mipmaps will fit more easily in the texture cache. This means texture data doesn't have to be fetched as often, reducing bandwidth usage and increasing application performance.

Does mipmaps increase FPS?

And a note too, mipmap will not decrease your fps, but anisotropic filtering will.

Is Mipmap good for performance?

Using mipmaps has two important advantages: Increases performance by massively improving texture cache efficiency, especially in cases of strong minification. Improves image quality by countering the aliasing that is caused by the under-sampling of textures that do not use mipmapping.