And is there any good evidence/sources for this? Surely both groups generally performed pretty extreme levels of backbreaking labour. I can't exactly imagine a slave slacking off while literally being whipped.
Also, even if feudalism made us more efficient at food production as you say* - did it not also make societies less effective at producing great works of infrastructure (roads being a good example) than imperial Rome? One just needs to compare the dark ages/low middle ages to the Roman era to see the stark contrast.
Isn't the entire narrative basically: Roman prosperity -> bad times -> rediscovery of classic values (renaissance) -> parity with the Romans 1000 years later?
Genuine questions, not trying to be combative. My history just might not be very good
* Also I'd be interested to find out if there were more famines during pax romana or in the period after the fall of the western empire..