Someone on the session used a good analogy, which I am going to butcher, but I internalized as:
SwiftUI is like a Makefile, and it keeps track of dependencies and recomputes things that need changing - if you dump everything into "global.h" and everything includes it, everything gets rebuilt.
The best way to reduce rebuilds is to narrow what you depend on.
I have notes for a blog post that I have been meaning to publish, but I feel like it should be more authoritative than just my lame anecdotes - and I also need to rewatch the video to use proper terminology before I post.