[jayd16]

Honestly, this is far too generous.

IMGUI is easy to implement so its the easiest to put in a custom game engine.

IMGUI is not particularly fast in practice. Unity's certainly isn't. However, they are simple, which approximates performance for small cases and it also lets you write your own tuned implementation.

Traditionally screen orientations for games were pretty simple so doing layouts in a single pass was feasible. These days, when you want to fit your game on every platform you need to do a lot of layout work. At that point you're just making an incomplete retained gui implementation.