Normal slowness of brain during any open heart surgery is not dangerous and is related to two factors - firstly, the anaesthesia and various other drugs that are given during the surgery and secondly, the lowering of the body temperature. However, if a person has any blockages in the arteries of the brain as a result of generalised atherosclerosis (deposition of cholesterol), then some times, this slowness of the brain can become a serious issue. The incidence of strokes, if that is what you are trying to find out, is 1-2% during open heart surgery.