This page details optimizations which are unique to iOS deployment.
Most of the functions in the UnityEngine namespace are implemented in C/C++. Calling a C/C++ function from a Mono script involves a performance overhead. You can use iOS Script Call optimization (menu: Edit->Project Settings->Player) to save about 1 to 4 milliseconds per frame. The options for this setting are:-
Unity iOS allows you to change the frequency with which your application will try to execute its rendering loop, which is set to 30 frames per second by default. You can lower this number to save battery power but of course this saving will come at the expense of frame updates. Conversely, you can increase the framerate to give the rendering priority over other activities such as touch input and accelerometer processing. You will need to experiment with your choice of framerate to determine how it affects gameplay in your case.
If your application involves heavy computation or rendering and can maintain only 15 frames per second, say, then setting the desired frame rate higher than fifteen wouldn’t give any extra performance. The application has to be optimized sufficiently to allow for a higher framerate.
To set the desired framerate, change Application.targetFrameRate.
If accelerometer input is processed too frequently then the overall performance of your game may suffer as a result. By default, a Unity iOS application will sample the accelerometer 60 times per second. You may see some performance benefit by reducing the accelerometer sampling frequency and it can even be set to zero for games that don’t use accelerometer input. You can change the accelerometer frequency from the Other Settings panel in the iOS Player Settings.