Hi SplineO,
Could you post your dxdiag? It could have something to do with your hardware setup.
With that said, standard Level Streaming is generally used for smaller scale levels than what you are describing above. What you may want to look into is the World Browser (now known as World Composition). There isn’t much official documentation on it yet because it just came out of the experimental stage with 4.3, but there is quite a bit of community info.
Details how to use World Browser:
Info about World Composition in 4.3:
https://forums.unrealengine.com/archive/index.php/t-14839.html
If you are 4.2.1 or earlier it is enable through the Editor Preferences > Experimental > World Browser. If you are using 4.3 it is enable through the World Settings > Enable World Composition.