Rendering a reflection in real-time involves in most cases re-rendering your scene once for every single camera that views the reflection and for every reflection in the scene. This can cause performance to drop significantly if too many reflections are added to a scene and many of them are visible at once.
While the actual cost of rendering the scene for a reflection is usually smaller than the cost of rendering the scene through the main camera there are some additional steps that you can take to improve performance even more.
Control Shadows & LOD
Use a lower LOD bias for your Reflection Renderer so that the reflections use lower quality models, reducing their performance impact. You can also change the Max. LOD value to ensure that only low poly versions of your models are reflected since, in most cases, players will not notice these smaller differences.
Additionally, you can disable shadows in the reflection unless completely necessary to reduce the amount of draw calls generated by every mirror in scene.
Lower the reflection’s resolution
If your game uses heavy shaders and post-process FX or has a lot of overdraw then reducing the resolution of the reflections may be helpful. Using a slight blur pass on them may also reduce the sharp edges and make them appear softer and more natural. On top of this, you can also set the reflection’s resolution to depend on the main screen’s resolution so that players using older devices and switching to lower resolutions get lower quality reflections automatically.
Limit the reflection’s framerate
Lastly, you can limit the reflection’s framerate to gain some additional performance. This however is not recommended unless your game runs at a very high framerate by default as with a lower framerate the reflections may seem too choppy producing an unpleasant effect in the scene.