Ionic mobility depends upon the size of the hydration sphere means greater the  size cation lesser the hydration sphere and higher ionic mobility. that is
$\text{Ionic mobility }\propto \frac{1}{ \text{Hydration sphere of alkali cation}}$
Size of hydration sphere: ${\text{Li}}^{+}>{\text{Na}}^{+}>{\text{K}}^{+}>{\text{Rb}}^{+}>{\text{Cs}}^{+}\text{}$
Ionic Mobility: ${\text{Li}}^{\text{+}}<{\text{Na}}^{\text{+}}<{\text{K}}^{+}<{\text{Rb}}^{+}<{\text{Cs}}^{+}$