When ${\mathrm{KMnO}}_4$ is added to oxalic acid, the decolorization is slow in the beginning but becomes instantaneous after sometime because

The ${\mathrm{KMnO}}_4$ converted into the ${\mathrm{Mn}}^{2+}$ which act as the auto catalyst. Initially the ${\mathrm{KMnO}}_4$ is first converted into ${\mathrm{MnO}}_4^{-}$ which makes the reaction slow after that when it is converted into ${\mathrm{Mn}}^{2+}$ then makes the reaction faster.