If you cook a big turkey in an oven it's gonna take like 16 hours to cook instead of 1 hour for a small chicken. There's just no getting around it. Let's see, human body is 60% water. If the average human body weight is 150 pounds, then it takes 92 MJ to evaporate the water inside the human body. That's how much it costs to go from 100 C water to steam, and doesn't include heating it up from room temperature. A kg of coal is about 20-30 MJ, let's say 30, so it takes 3 kg of coal to evaporate all the water inside a human body. That's the lower-most bound on how much energy it takes to cremate a human body. I don't know how much energy it takes to burn the rest once the water is evaporated. Realistically given the inefficiencies involved it probably takes 10x that, so 30 kg of coal to cremate a human body. Basically they'd need thousands and thousands of tons of coal to cremate a million humans.