World composition limits

Hi! I know that I am probably pushing engine past its limits, but theoretically world composition should allow world of any size. I have 2200 levels 5x5km each - they are some static meshes. Now, after I clicked ‘play’ inside editor, it simply stuck on processing something for hours, I had to alt+ctr+del quit it after tired of waiting.

Any idea, on what editor is actually calculating all that time? Maybe if I have some knowledge, I’ll know how to work around it. As every level has 5 km offset from each other, Editor should had streamed just few levels and be OK with it. Maybe it is caching something for the first run or something like that?