A contact rescue, swimming to the victim without a safety line, may take little or no time to set-up but it also carries a great deal of risk. If the person is conscious and can move, there are so many other better alternatives that can be just as fast - remember RETHROG, this is the GO part and the last one on the list. Live swimmers can panic and unless you have some lifeguard training, both of you can end up drowning. The simple part is getting to the victim. Swimming both the victim and yourself to shore isn't so simple. Sometimes there isn't any great alternative or rescue time is very limited. A good example is an unconscious swimmer reasonably close to shore.