Straight from the division chair of the local morticianry science program where I work...
There are lots of different factors. Overweight people burn faster than underweight people. The more muscle mass a person has on them the longer it takes. Age and the natural characteristics of the skin matter. (This is kind of gross so, but this is what he told me and if I can't share it with you guys....
Spoiler:
The skin chars immediately when exposed to the heat. It then splits or bursts and then sloughs off in big chunks.)
It takes longer to
safely cremate an obese person than a regular sized
joe....or joe-ette..whatever.
Overweight folks tend to burn faster and burn hotter, which means professional crematoriums have to take it slower to prevent accidents and such. If you count the time it takes to pulverize the bone as part of the whole cremation process and not just the time actually spent in the oven, then it's even longer. Big people = Big bones to cart all that big person around for 50 some odd years.
So....they don't naturally burn slower.....crematoriums purposefully burn them slower for safety reasons.
Quick Google search...