And this is how to get rid of the originals
Here, I put 3 ‘walls’ into the level and instanced them:
The system has copied the location rotation and scale. I know you can’t tell from there, but this is one thing, I can’t click on those walls separately, it’s like foliage, one big thing.
Then you can just save the level. BOOM - much better performance.
Tell me how you do 
PS: Try it on larger and larger amounts of meshes. I think it might take a few minutes to run on 10,000 or so…
PPS: Please also see what I added at the top. You need the blueprint to save the instance info in the save game. Otherwise everything disappears when you click save…