Say a program runs and results in $T_{1} = 157s$ then is modified and runs in $T_{2} = 16s$.
What is the correct way to display (in %):
- How much faster is $T_{2}$ than $T_{1}$?
- How much of an increase is the completion time of $T_{2}$ than $T_{1}$?
I imagine them to be read as:
- $T_{2}$ is __% faster than $T_{1}$
- $T_{2}$ was an increase of __% in completion time
With the latter being something along the lines of "... $4000$% ..." or "... 32x ..." or so.
A way is use, with reference to time, the ratio
$$p_T=\frac{T_1-T_2}{T_1}=\frac{141}{157}\approx 0.898=89.8\%$$
or as an alternative with reference to speed
$$p_S=\frac{\frac1{T_2}}{\frac1{T_2}}=\frac{T_1}{T_2}\approx 9.81=981\%$$