I second Peláez's answer. In addition, I want to explain this mistake of yours: “I assumed that 10% faster means it takes 10% less time (60 seconds down to 54 seconds).” 10% normalized = 0.1 , oldTime=60 , and the correct result is not 54:
$newTime = oldTime/(1+0.1) = 54.54545454545454\dots$
$\neq 54 = oldTime\cdot(1-0.1)$.
If $X\to 0$, then the difference between formulas $\to 0$, so in calculations $\cdot(1-X)$ is often used.
But the following statement is correct and precise: 10% faster means that something moves 10% further.