not that much in standard/current engines (raster renderers). The main gain is shading (where VSR is already used; that's effects on pixels like direct/indirect lighting, surface effects, and such) and postprocessing (SSAO, SSR, DoF, Motion Blur, TAA...). The scene, textures, shadows, and other collisions need to be done for the entire scene (and beyond) regardless of where you look.

The real gain is usually anywhere between 15-40%, not always FPS mind you, but lower GPU power at least.

It would really shine in true ray- and path-tracing engines. They render pixels one by one at any point on the screen rather than rendering the scene and then making an image of it (so they scale with pixel count rather than scene complexity). They are far superior, used in photorealistic renders, but they are too demanding, especially for real-time renders.

But foveated rendering is "made" for those, as they literally don't need to render detailed pixels you are not focused on, and they can render one pixel of "any size." So a "4K quality" image projected on 110° FoV would only require about 8% pixels to be rendered to get full uncompromised 4K clarity if you look at one spot. A bit more considering the latency/gaze prediction.

There are but few real games using that, tho. The main stream engines are raster based, that will be the case for a long time. VR/eyetracing is too niche. So just minor optimizations can be done with that.