If you are flying one of those ready to fly pod and boom planes, slowing down the motor is not going to make the plane fly slower. The only way to achieve what you want is to make the plane lighter or increase the wing area.
These planes respond to an increase of airspeed by automatically pitching up resulting in a higher rate of climb which in turn keeps the airspeed down. Turn the motor off and the resulting loss of speed causes these planes to automatically dive which causes the airspeed to stay about the same. This is a natural charactoristic of trainers in general.