The are formed by mutual sharing of equal number of electrons between two atoms

The are formed by mutual sharing of equal number of electrons between two atoms. A covalent compound contains the strongest types of bonds (covalent bonds) and so it takes a lot longer for them to be broken because they are so strong.

The reaction between covalent are usually slow because the rearrangement of electrons and nuclei takes place slowly at room temperature.

Another reason is that reactions of covalent are slow because the reaction involves first breaking of existing bond and then formation of new bonds.