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So here is a question that I've asked and received no good answer. Why size of the salt doesn't matter? Wouldn't salt be used in every hash computation? Wouldn't large (megabytes) salt slow down this computation and require more memory to perform it? I'm not advocating the use of large salt as opposed to specialized functions, I'm just curious as to why it doesn't work. The article doesn't explain that.

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It's not that the size doesn't matter; it's just that it's not as significant. It becomes very, very hard to compute rainbow tables after just a few random bytes. No matter how long the salt is, it doesn't do anything to prevent somebody from trying to guess the original input for a given digest using a brute force approach, though. So usually the salt has less than e.g. 256 bits of entropy just because it takes up less space.

Sure, a very large salt might slow down the first iteration a little (but not necessarily subsequent ones, and it wouldn't require more memory, at least with most hash functions), so you're almost always better off just stretching the key--then you save the storage costs too.

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