It makes it easier to use whole numbers..
Lets say X = 100 and Y = 50
You're saying "What percentage faster is time 1 to time 2?" This means, with respect from time 2, how much faster is time 1... Again, using time 2 as the reference point, how does time 1 compare.
So for this, you would use: T1 / T2 = (100 / 50) = twice as fast = 200%
In your case above, X < Y so it would be a percentage less than 100%. Roughly 44.9% faster.
