As to what happens when the attacker gets it, that's a complex question.

As per the HKDF paper, bad things can happen if the attacker can influence the source while knowing the salt. For more information, see  B. Barak, et al.'s work on t-resilient extractors, e.g. http://www.cs.tau.ac.il/~tromer/papers/rng.pdf.

It should not be possible to do a classic dictionary attack when knowing the salt, not because HKDF is slow, but because low-entropy inputs to it are a bug. However, software has bugs, so perhaps we should document that that is a bug, and document that that is maybe not the only bug.