While adding a Planar Reflection makes the water look more lifelike, the impact it has on performance is quite small when compared to the Kite Demo. If you take a look at the image on the left hand side below, Planar Reflections Off, you can see that without Planar Reflections the scene is costing around 11 ms to render. If you take a look at the image on the right hand side, Planar Reflections On, you can see that with Planar Reflections cost around 1.67 ms to rendering bring the total time to render the level to around 13 ms.
Planar Reflection Off
Planar Reflection On
Click for full image
Click for full image.
The reason that Planar Reflections only cost 1.67 ms to render in the Elven Ruins map versus the 23.07 ms that it takes to render the Planar Reflections in the Kite Demo is due to how the Static Meshes and Materials that are used in the Elven Ruins were built. Since the Elven Ruins map and its content was designed for mobile devices, close attention was paid to the amount of triangles and Material instructions each asset used. Because of this, when Planar Reflections are enabled in the Elven Ruins map, they will cost less as the assets that are used in the map are greatly reduced in complexity and size when compared to the Kite Demo assets.