In this case where the building is flat the spline is kinda useless
In practice there could be a better possibility but that needs some work to prepare - your own custom BP that will lay it around, it’s not that hard to create, more time consuming
Normally buildings meshes come already with windows xD
And it makes sense as having multiple actors separately instead of having a full model (that you can also modify to be a less detailed mesh by distance) is just power consuming to update them all at all times
The less actors the better, I know that because placing 10k static mesh planes (yes, a single flat square) drops to ~10fps
But after I used an Instanced Static Mesh I used only one actor that I drew over the viewport a fractal - I had still over 20fps with over 200k of those planes
The problem was so called “draw calls” and you may encounter that too if you will overhaul the map with actors that are totally unnecessary instead of using proper meshes