It's quite difficult to find the optimal speed. Actually you don't need to be a perfectionist when you want to remove the extra body mass. Just keep in mind that your goal is to lose weight. The optimal loss rate for me is about 3 - 4 kg per month. This will avoid all related unpleasant side-effects of the rapid weight loss. If you exceed this value than you risk:
vitamin and mineral deficiency - of course with proper intake of tablets you can minimize the effect but not to avoid it.
lack of energy and low endurance levels - your body will enter in starvation mode or even in survival mode;
instead of fat you can lose muscle mass - and this is certainly something that you don't want to happen. First muscles burn more fat (even in relaxed more) and second the body will try to preserve the fat. Under the stress of the starvation the body will cut the energy consumers and will keep the energy sources. You are going to do the same, aren't you?
Possible problems with blood pressure(low blood pressure). The process of weight loss is related to better control of the blood pressure. If you speed up this process you will go from one extreme to the other. So be careful and more balanced here.
loose skin - pretty unpleasant problem. Fixing of this problem usually is expensive, with doubtful effect and related to medical surgery;
gaining again the lost weight plus more because you are lost muscles;
These are some of the problems related aggressive weight lost regime. Be sure that a high speed will always be punished at the end!
So my advice here is play smart and don't solve the problem ending with more problems!