Using image effects (in this case Grayscale Effect) together with the "Depth only" camera clear flag drastically reduces FPS on the device. The application has 60 fps with "Solid Color" clear flag, however with the "Depth only" clear flag it has only 3 fps. It is a regression on some of the devices. With Unity 4.2.2f1 the fps dropped to 15-25 on the Google Galaxy Nexus, but with Unity 4.3.4f1 it drops all the way down to 3 fps. On the other hand on Google Nexus 5 the fps drops to 3 in both Unity 4.2.2f1 and 4.3.4f1.

Reproduced on:
Google Nexus 5 with Android 4.4
Google Galaxy Nexus with Android 4.3

Reproduced with:
Unity Version 4.3.1f1 (cebd4d9b5122)

[Edit]
The workaround the user found in this case was to use a single camera and to render the image as a quad. By using a single camera, they avoided the 'Depth-only clear' which is slow on this hardware.

Resolution:
This is expected behavior. Clearing depth only means that the GPU must do framebuffer unresolve in order to retain the previous color surface contents.