Why are you blaming the engine for being slow, instead of the developers.
All the engines run perfectly fine until you put something poorly optimized into it
that's not really how it works
the engine's gonna be the part that requires the most. it has to render everything, figure out how to light the scene, do the... physics, and everything like that
relative to all that, the gameplay doesn't take much to run
however, Unreal is a great engine, and it does everything well. so it's not that engine's fault tristan can't run Unreal Engine games