Sudden weight-loss means you have too much skin for what's underneath, which leads to unsightly sagging. You have to give the skin time to adjust to what's there, to resize, as it were.
I've seen pictures of what happened to people who lost weight too fast - elephant ear arms, basically, it isn't pretty and there's nothing you can do about it. Surgery leaves terrible scars. :(
Just recently there was a question in Ask CureZone team from someone who lost a lot of weight and has a sagging tummy. Since you're limiting calories, the skin doesn't have all the nutrients it needs to be as elastic as it needs to be to properly adjust. It's slow to adjust anyway, but this is compounded by not having essential oils and vitamins. So slow weight loss is best for a LOT of different reasons. :)