Can you link to something authoritative so I can cure my ignorance?

Sorry, I didn't find anything definitive either. However, it follows from the normal use for ratios less than unity. The only difference is the magnitude. Taking "two times" to be equivalent to "200%", and "1/2 times" (or simply "1/2") to be equivalent to "50%":

50% as fast (as the original) = 1/2 (times) as fast = 0.5 * original speed

100% as fast = one times as fast = 1 * original speed

200% as fast = two times as fast = 2 * original speed

50% faster (than the original) = 1/2 (times) faster = (0.5 * original speed) + original speed

100% faster = one times faster = (1 * original speed) + original speed = 2 * original speed

200% faster = two times faster = (2 * original speed) + original speed = 3 * original speed

The expression has two parts. The first can be either "X%" or "X times", both relative to the original amount. If the second part is "as fast" or "as much" (etc.) then this is the final result. If the second part is a relative term like "faster" or "more" then this implies addition, and the first amount, after multiplication, is the difference between the result and the original amount.

Few would disagree with the statement that "50% faster" is equivalent to "150% as fast", and not "50% as fast", but for some reason many become confused by "200% faster" when the formula is exactly the same.