No performance gain with FFR - how is that possible?

Hello, Unity developer here, I know this is old, but hopefully this can be useful for someone. FFR will greatly help if your scene\app is fill bound on performance (basically means at the fragment shader stage, where the gpu calculates the color of the pixels to fill the screen, shaders that uses depth buffer like water shaders for example, could benefit from it). If your problem is on the vertex shader stage (generally caused by too many polygons per frame) it might actually have an negative impact in performance. (source: self experience and https://developer.oculus.com/documentation/native/android/mobile-ffr/?locale=pt_BR#foveation-example-images).

That said, I actually had issues on a project trying to enable FFR, in my case I am using Unity with URP and I am changing the renderer settings based on the scene that is loaded and changing the resolution seems to mess with FFR, it ends up resetting to level 0 everytime.

Related to not being able to see the difference on screen: The best way to know if FFR is enabled or not (that I know of), is using Oculus OVR Metrics app. It will give you a lot of useful data at runtime of any app running on the oculus. It is really a must have tool if you want to debug your app. That said, also, if you are using Vulkan, you can enable Subsampled Layout, which diminishes FFR artifacts (it may have a performance impact though). (source: About the Oculus XR Plugin | Oculus XR Plugin | 2.0.0-preview.5 (unity3d.com)).