This is a common misunderstanding (even with the caveat that you are talking about cryptography and the author is not). There is no reason that the hashes used

This is a common misunderstanding (even with the caveat that you are talking about cryptography and the author is not). There is no reason that the hashes used for e.g. signing a document should be slow. If you have an application where you need the operation to be slow you can construct an iterative version; you'll want an adjustable work factor anyway.