Some ideas to help.
- Try looking at the scene in the editor with shader complexity turned on.
View Modes | Unreal Engine Documentation - You have 171 million primitives in the scene. That feels way too high. Can you reduce the polys and texture in far away objects some?
- The two very expensive things that are the usual culprits are transparencies and lights. If transparent surfaces (like with an alpha of 0.5) overlap the rendering gets very slow. Too many light can slow rendering way down in particular if they cast shadows.