Following the track of “the light is the problem” I have a few questions about it:
How is your Light Importance Volumes around the level?
Your stationary lights, can you set them to static lights, just to test it?
About the materials, can you use a base material instead to check if you got any improvement.
Also, can you profile your scene rendering and post a picture of it statics? (GPU Profiling | Unreal Engine Documentation)