We have studied the frequency and temperature dependence of the dielectric constant parallel to the ferroelectric c axis in the ferroelectric phase of $\sf\small KH_2AsO_4$. The dielectric dispersion found in the high-frequency range (1 MHz-1000 MHz) could be described by a Vogel-Fulcher-dependent Gaussian distribution of relaxation times, similarly as in $\sf\small KH_2PO_4$. However, the use of a Vogel-Fulcher law is less convincing than in $\sf\small KH_2PO_4$. Below the freezing temperature $T_f$ the relaxation times strongly increase resulting in a decrease of the domain wall mobility. This may be due to the non-linear superposition of the piezoelectric resonance and the relaxational dispersion.