The English language defines the denominator in your equation as the object of the sentence.

If you have 100 blue smarties in a bowl, and there are 20% more red smarties than blue, then you have 120 red smarties. There is no ambiguity.

I think that percentages shouldn't be used for things like this.

Show an absolute time, how many units/second of work.

percentages just (intentionally??) confuse things.

the problem is that %faster and %slower are not equal