It does have to traverse the complete filesystem to calculate the sizes. That is how most filesystems work (files have sizes, folders do not - so a folder size doesn't exist and is just a sum of whatever it contains)
Interesting, I thought it uses the Spotlight index to get this information and that the index already contains these aggregates, but you're probably right considering that it works even in folders that I've explicitly excluded (I wonder how many millions of node_modules folders are indexed for no reason).
