Why Do Languages Change? (2010) by R. L. Trask. p. 13.
Changes in pronunciation can happen with considerable speed. Consider /hw/. Historically, English had a number of words beginning with the sequence of consonants /hw/, curiously spelled <wh> since the Middle Ages. This sequence was pronounced very differently from plain /w/, and so whine sounded different from wine, whales from Wales, which from witch, where from wear, wheel from weal, and so on.
Why wasn't /hw/ spelled, more straightforwardly, <hw>?
I'm uncertain if this answer at ELU answers this question.