The collector plate in an experiment on photoelectric effect is kept vertically above the emitter plate. Light source is put on and a saturation photocurrent is recorded. An electric field is switched on which has a vertically downward direction.
when electric field is switched on a force acts on the photoelectron which is opposite to the direction of electric field which increases the velocity of electron hence kinetic energy.
The stopping potential and the threshold wavelength depends on the metal not on external conditions.
And in saturation condition, no current is increased by increase in the speed of photoelectron.