Did a test with some of the foods you can give to the thralls. This is the result:
Food is in this order:
Enhanced Gruel, Roasted Haunch, Gruel, Shredded Roast, Grilled Steak
They ate the Grilled Steaks faster than the other foods in the test. Both Gruel’s and the Shredded Roast were not touched in this test.