Time is the measure of computer performance. The speed of processor when executing instruction is measured by time:
$$ \textrm{Performance} \propto \frac{1}{\textrm{Time}} $$
The basic performance equation in terms of calculating CPU time $T_{code}$ is:
$$ T_{code} = \frac{1}{f_{CPU}} \sum_{i=1}^n (N_i \times CPI_i) $$
where $N_i$ is the instruction count, $CPI_i$ is the clock per instruction, and $1/f_{CPU}$ is the clock cycle time.
Performance of two applications — e.g. $A$ and $B$ — can also be compared. We can say that $A$ is $n$ times faster than $B$ if:
$$ \frac{\textrm{Performance}_A}{\textrm{Performance}_B} = \frac{\textrm{Time}_B}{\textrm{Time}_A} = n $$
$$ \textrm{Performance}_A = n \times \textrm{Performance}_B $$