Somehow I always had the impression Unity was a heavy C# moloch and Unreal was a fast C++ racecar.
Interesting.
Interesting.
What really makes the difference is that Unity has light-weight rendering pipelines available, running on older GPUs. UE4 just ditches those devices.
It's not about performance in 99% of frames, it's about not dropping frames when the GC kicks in and needs more time than your budget accounts for.