That's caused by how lighting and shading is done by the engine. This type of level setup would cause problems with shading in any other engine as well. You have all these fragmented models all over the scene and the engine has to draw each of them separately and shading them before they appear into your screen. Look up "drawcalls" and understand how they work.
UE4 isn't just a magical thing that makes everything look awesome whatever you do, it still has a lot of limitations which needs to be taken into consideration.
In this situation either hide your seams behind pillars or make your ceiling, walls, whatever out of single mesh.