The old system would stack up draw calls like nobody's bussiness, doubling drawcalls if you used the built in layout tools. OnGUI being called period would be expensive, even when you had properly implemented your rendering logic, so you’d have to disable the entire game object when you didn’t want to draw.
The docs for mobile literally used to say not to use the OnGUI stuff during gameplay.
... but IMGUI is on the way out there too: https://docs.unity3d.com/Manual/UIElements.html
The old system would stack up draw calls like nobody's bussiness, doubling drawcalls if you used the built in layout tools. OnGUI being called period would be expensive, even when you had properly implemented your rendering logic, so you’d have to disable the entire game object when you didn’t want to draw.
The docs for mobile literally used to say not to use the OnGUI stuff during gameplay.