When we move down the group of periodic table, there is an increase of electron of which in turn increases the number of orbitals for the electron to occupy.
Also that lead to reduction of radius from the outer electron to the nucleus. I so when atom have much electron in its orbit, the rate at which they will ionize decrease and the rate of ionization energy increases. The atom at the top of the periodic table have less electron as such the electron is loosely spread in their orbitals as such have high ionization and low ionization energy.
On these explanation we can conclude that going down the group in the periodic table, the rate of ionization energy increases while the rate at which ionization take place is reduced or decreased.