Recent joiner here, but this is my area of interest. In modern times you can use technology to make "idle time" work for you. A slave, even in storage, has 24 hours in the day like any free person. You can't have physical trainers the whole time, and the slave has to be permitted a certain amount of sleep. How to fill the 24 hours for efficient processing?
Sensory input. Assuming the slave is in a containment unit of some kind, control of the lighting and sound must be outside its control. They must be unable to communicate with other slave units in storage. So the cage is soundproof. It is a single lighting source and a single sound source, like a loudspeaker. The slave may not be aware of it, but a monitor camera and microphone are desirable too.
When not in training, sleep periods are strictly defined, and may include elements of sleep deprivation. Lights go off and on to indicate sleep times. Slaves are monitored during sleep times and may be punished for not sleeping.
Automated feeding - a nozzle for water. Voice recognition, like Siri, can be used so that certain key phrases must be spoken by the slave before water is granted. For example, it must say "This slave begs for water" a certain number of times, before being allowed to suck at the nozzle. All possible using modern tech. Other phrases may be required before feeding.
"This slave begs for water"
"This is my new life now"
"I love to obey"
"Thank you Master"
"I love being a slave girl/boy"
As a bonus these responses can be recorded and then fed back subliminally into the sounds of the confinement unit. The slave hears their own voice making these powerful statements. Maybe fed back during sleep periods.