(Something similar to this post was originally written by Boris Zbarsky on his blog, but I can't find the post now.)
If you have a function which takes up 50% of our time, and you optimize that function so that it takes 30% of your time, by what percentage did you reduce overall time and by what percentage did you reduce time in that function?
The tricky bit here is that the 100% reference value is different, because it was also reduced.
|<------------------------------- 100ms ------------------------------->|