My point was that different designs can be more or can be less scalable; poor designs require you to change lots of things to make one functionally-distinct change. There was no "it" (specific class of architectures) I was calling O(n^2), but an example case might be where all the interfaces assumed p fields, each with q bytes, then having one component pass more information would require updates to all components O(n) rather than just that one.

I don't see how describing an O(n log n) system sheds further light on any of this.