Best way to convert BP Actors into level actors?


First, I wonder, is it a right way, we choose, to creating a large City scene?
The way we choose is to do it with a modular peaces of geometry, instead of doing each building with one mesh, material and texturing.

Second, since we are already do so, I suppose that we aren’t using a right technology for that.
However we construct each building in a blueprint viewport, so we have a blueprint for every building(not speaking about benches and other environment components that has repeating peaces)

Now when each blueprints in its place on the scene, editor is slowdown…so the game would be too.
After small test, I figured out that the level actors are less expensive then blueprint actors…
Don’t understand, why, but it is fact - scene FPS speed up when I “import to level” by “level actors” instead of “blueprint actors”.

So the question is: how can we convert BP Actors into Level actors in the editor, rather then rebuild the entire scene?