In the nitration of benzene, the electrophile is the nitronium ion ($NO_2^+$), which is generated by the reaction between concentrated nitric acid and concentrated sulphuric acid: $HNO_3 + 2H_2SO_4 \rightleftharpoons NO_2^+ + 2HSO_4^- + H_3O^+$ The addition of $KHSO_4$ introduces a large amount of $HSO_4^-$ ions into the mixture. According to Le Chatelier's principle (common ion effect), the increased concentration of $HSO_4^-$ shifts the equilibrium to the left. This suppresses the dissociation of $HNO_3$, leading to a decrease in the concentration of the electrophile, $NO_2^+$. Since the rate of electrophilic aromatic substitution depends on the concentration of the electrophile, the rate of nitration will be slower.