Took us a while to find out why and where.
In short, every instance of the OnGUI method generates 336 bytes of garbage and take 0.12 ms of CPU every frames.
This is because Unity assume your OnGUI would like to use GUILayout, and therefore creates a new GUILayoutGroup object every frame.
To prevent this from occuring, you should do this;
private void Start()
{
useGUILayout = false;
}
Scaleform uses the OnGUI method to trap input, but does not use it to display anything, and surely not any GUILayout.
You should consider adding this flag to your SFCamera if you like not generating useless garbage every frame.
From Scaleform\Unity Development: