Like stated above you have to really optimize things, without going into source code optimization a few things you can do at the editor/BP level are:
- use soft references wherever possible
- use LOD
- miinimilize poly count and texture size as much as possible
- use level streaming
- use as little actor ticks as possible
- bake static shadow
There’s is more you can do at the BP and editor level but these are a great starting point