[nequo]

Immaterial to the substance of your point but a 50% speed-up is an improvement from 40x to 20x, not to 30x.

        X’  X’/Y   X’/Y
  0.5 = – = –––– = –––– 
        X   X/Y     40

        X’
    =>  – = 20
        Y

[kzrdude]

Depends on how you define speed-up. This is the most natural definition for me:

As a baseline, my code can do 100 operations / second.

With 50% speedup (+50%), my code can do 150 operations / second.

If C was 40x faster than the baseline, it could do 4000 operations / second. After the speedup C can still do 27x as many operations per second. (4000/150 = 26.66..)

[nequo]

Well, that’s true. The good old miles per gallon vs. liters per 100 kilometers conundrum.