There is evidence that drinking cold water or ice-slush beverages improves endurance and overall performance in hot and humid weather. During strenuous exercise especially while participating in competitive sports in hot weather, core body temperature rises accordingly. Body metabolism increases and more calories are burnt for similar performance. Increase in the core body temperature also affects cardiovascular function, contributes to fatigue, causes dehydration and also impacts electrolytes balance in the body. Studies show that consuming cold beverages drops core rectal temperature of the body by few degrees supporting thermoregulatory effects in the body. Drinking cold beverages than fluids at room temperature definitely helps improving performance. No difference was found in the performance between drinking cold beverages and ice-slush.
There are no good studies that can demonstrate if drinking cold beverages are healthy when we are not exercising or not in hot and humid weather. However, physiologically, drinking cold fluids still brings down core body temperature even if we are not exercising. According to Ayurveda, a 5000 year old ancient Indian medicine, drinking cold water results in improper food digestion and further digestive problems. Drinking one or two glasses of cold beverages probably does not have a serious effect on overall health of the individuals but drinking large quantities like more than a liter of cold beverages is probably not healthy. Overall effect also depends on how fast we guzzle these drinks.