Not into your code, but sounds promising just by the numbers. Would this also affect overall performance or just wall updates? The street tool could need some love as well. Placing down streets cuts my FPS in half. Settlements above 500 become very laggy and with 700 it’s unplayable, at least for my setup. What I noticed is that my hardware is not even used efficiently. CPU at max 50%, GPU bored to death. So either there is some terrible bottleneck I can not see in my graphs or the hardware is not fully utilized at all.
Don’t get me wrong. I love the game so far, you are doing an excellent job and I have trust in Crate fixing this completely. Just curious, willing to help and maybe a little bit addicted to continue with late game. 
And here is an interesting finding regarding performance potential: Let's talk about performance - notifications increase FPS - 0.7.5f