why a very slow process can be treated ad isothermal and a rapid process adiabatic

Because in an very slow process, temperature remains same, hence the name ISOTHERMAL!

But when gases expand or contract quickly they get cool or hot respectively! So the name adiabatic!

            A very slow process could be treated as the Isothermal process, because due to the lapse of time the temperature gradient becomes lesser and some sort of steady state is obtained by this heat transfer could be done without change in the temperature.  (T = Constant)

           A rapid process could be treated as an Adiabatic process since the process is soo quick where there is no time for heat transfer to take place.  (Q = 0)