[tokyodude]

I think you're mis-understanding why Unity switched. They only switched their in game GUI system. The entire app of Unity itself uses an ImGUI system and it is massively more complex than any Game UI and runs just fine.

The reasons they switched the in game system are many but they have nothing to do with performance.

[BoorishBears]

The editor has none of the constraints the games made with it have... like running on mobile and maintaining 60 FPS while doing it.

... 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.