More ions form pairs when concentration of acid increases, does that decrease the speed of reaction?
In general, the conductivity of any liquid increases with the concentrations of its ions, and decreases with their dimensions. In pure sulfuric acid, the only ions existing are $\ce{H3SO4+}$ and $\ce{HSO4-}$. They are big and their concentration is low. So the conductivity of pure sulfuric acid is low. But when you add some water, the $\ce{H2O}$ molecules are totally transformed into $\ce{H3O+}$ and $\ce{HSO4-}$ ions which are numerous and sometimes small (for $\ce{H3O+}$). The conductivity must increase.