Increase speed

Travelling time $t$ is inversely proportional to speed $v$ or $t \propto \dfrac{1}{v}$ . Therefore,

$\frac{t_2}{t_1} = \frac{v_1}{v_2} = \frac{v_1}{1.25 v_1} = 0.8 \\ \Rightarrow t_2 = 0.8 t_1$

The ratio of time saved:

$\begin{aligned} \frac{t_1-t_2}{t_1} & = \frac{(1-0.8)t_1}{t_1} = 0.2 = \boxed{20} \% \end{aligned}$

Since the distance is the same,

v $t_{1}$ = $\dfrac{5}{4}$ v $t_{2}$

Which means that

$t_{2}$ = $\dfrac{4}{5}$ $t_{1}$

The time is shortened by $\dfrac{1}{5}$ or 20% of the time of the first trip.