itch.io

I've tried 10 times to attach a image and some references to it but itchio kept bugging, but basically i don't think it has to do with the code, but the engine's/scene's lighting and shadow settings. 

For a game like this you could turn down the resolution of shadowmaps and still achieve a nice effect with some performance boost, but i'm not a UE dev so i can't guide you for that, sorry.
Also, since you have a lot of static stuff in the scene, maybe you could use mixed shadows instead of fully real time illumination, so you'd bake the lighting for all objects that are meant to stay in place, but still have real light illumination and shadows of moving objects affect them. 

Another setting that comes to mind is the amount of light bounces, and toning that down can greatly improve performance. But as mentioned, I'd be best if you can find some tutorial or someone to guide you about how to achieve that in UE, since i've no clue where even the settings are. 

And last but not least, in case UE has a equivalent to this in Unity, Unity has a checkbox in the gameobject properties that allows you to mark them as static. Any object in which I enable that for, will be skipped by the physics engine, also helping with performance. You may want to look into this as well, just in case it has such an option.

I hope it helps, cheers :)