Unity and UE4 are very flexible, very generic, friendly, very powerful & very portable. But this has come at price of performance where they pale against a custom tailored engine (by several orders of magnitude, I'm talking between 4x & 10x difference)
That's really interestering. Could you give an example or two where another engine is that much faster than Unity or UE4?
Fox engine seems superior to both Unity and UE4 performance wise.