Why do slaves have to be restricted to physical labor? Why not have slaves toil away in telemarketing or retail (feel free to hit the store employee if he gives you lip). Slaves in general would be useful for jobs that most people CAN do but most people do not WANT to do.
I imagine slaves would be used mostly in such jobs. Basically, i would take a look at the low payed sector today and ask: Which jobs wouldn't also or even work better with slaves? Waiters, service personel etc. that perfectly incorperate the brand and are bred/designed to be great at their job etc.
Would be a trade-off between productivity peak and productivity lifetime...That's why I said little to no rest. And terrible is relative. Let me rephrase it: they have worse working conditions and less rest as long as it let them produce more.