I understand that formants represent the resonances of the vocal tract and that the frequency of a formant is determined by the shape and size of the vocal tract.
If we speed up the speech waveform, that would be comparable to shortening the vocal tract. As I understand it,the sound itself would stay the same, let's say, an /a/ stays an /a/ but it would sound like it was spoken by a person with a shorter vocal tract, i.e. the frequency of the sound will increase as the higher frequencies are boosted due to the shorter resonance body.
Consider the following problem. We have the fricative /sh/. When we speed up the waveform, there is more energy in the higher formant frequencies of the sound and /sh/ is perceived as /s/. So, why is it the case that /a/ stays /a/ and /sh/ turns into /s/. Is this just an exception to the rule, or am I confusing something here?