I suppose there are degrees of painlessness. Pain receptors communicate with our brains to warn of us about damage that is occurring to our bodies. This pain makes us uncomfortable so we can react and survive. Cutting, traumas, and other methods of CTB will generate this type of pain.
Hunger will not necessarily activate any particular pain receptor, but I assure you that it's not without discomfort. In my depressed resolve, I once went 3 days without water and 10 days without food. I was weak, couldn't think straight, and was both physically and psychologically miserable. (I weigh around 140.) My resolve evaporated on day 11.
Observe hungry populations, especially children, in Somalia, Ethiopia, and other impoverished nations. They are not weeping in pain, but they are miserable. Starvation takes some time because your body will draw nutrients for energy from muscles, organs, and any other place where protein can be found. Your brain will allow you to be aware of the wasting effect.
The will to survive kicks in rather quickly during crisis. Imagine the battle you have to endure for 20-60 days, depending on many factors, without eating for your body to finally run out. No thank you.