Level streaming IS improving the performance for you, that MS wasn’t loaded until it needed to be. So, things are better while it’s not loaded.
Level streaming is how to do endless open worlds, if you want more control than world partition. You get to load and unload stuff while the player is moving around the world without them knowing.
If you’ve got a level that’s heavy to run, naturally, level streaming won’t make any difference to that particular area. It’s not magic 
Rereading your post, when you have already loaded the MS stuff, but are not in the room with it, the engine is not drawing it, because it’s occluded. So the performance would be fairly good, but maybe not as good as if the room was totally empty.
Yes, the engine automatically hides stuff we can’t see, but we can’t lean on that system totally and just fill the world with 400 million MS meshes. I’m sure you can see, that won’t work