Ice is water you have already added
A glass filled with ice and then topped with tea is a glass that will end up containing a great deal of melted ice. If the tea was brewed at drinking strength, the finished drink is a diluted version of it, and the usual verdict is that iced tea is watery. The fix is not stronger-tasting leaf or a longer steep but brewing for the volume of liquid you actually pour, treating the ice as the rest of the water. Flash-chilled iced tea works on exactly this basis: roughly double-strength liquor poured over roughly the volume of ice that will make up the difference.
Why ice cools so effectively
Melting ice absorbs a large amount of heat without changing temperature — melting a gram of ice takes roughly as much heat as cooling a gram of liquid water by about eighty degrees. That is why a glass of ice can drop hot tea to cold almost instantly and why the ice level barely seems to fall while it does so. It also means the cooling is nearly finished before the dilution is: the drink chills fast and then keeps getting more watery for as long as any solid remains. Serving over a smaller quantity of larger cubes slows the second stage without weakening the first.
Speed of cooling changes the drink
Chilling quickly rather than slowly is not just convenience. Aromatic compounds continue to leave a hot liquid as it stands, so tea that cools slowly over half an hour arrives at the same temperature with noticeably less aroma than tea that reached it in seconds. Rapid chilling also passes quickly through the range where hot tea goes hazy. The practical consequence is that pouring hot tea into a jug and putting the jug in the refrigerator is the worst of the available options, and it is the one most people reach for first.
The haze, and what it is
Strong tea that cools slowly turns cloudy. This is a physical change rather than spoilage: compounds that were comfortably dissolved at high temperature are less soluble as the liquid cools and come out of solution as a fine suspension. Stronger brews haze more, because there is more dissolved to come out, and hard water makes it worse. Chilling fast limits it, which is one reason the sweet tea method dilutes with cold water before refrigerating. A hazy drink is a cosmetic problem, and heating it gently redissolves the haze, which is a useful trick and a poor drink.
Sweetening cold drinks
Sugar dissolves far more readily in hot liquid than in cold, which is why sweet tea is sweetened as a hot concentrate and then diluted rather than stirred cold. Stirring granulated sugar into a cold jug leaves a gritty layer at the bottom and a drink that tastes progressively sweeter as it goes down. If a cold drink needs sweetening after the fact, a syrup dissolved separately is the only approach that mixes properly. This is a general property of solubility rather than anything about tea, and it catches people every summer.
Three cold drinks that are not variants of each other
Flash-chilled tea is hot-brewed, aromatic and made in minutes. Refrigerated cold brew is a slow low-temperature extraction that is sweeter, softer and almost incapable of bitterness, and it takes hours. Ice steeping is colder still, yields a very small concentrated volume, and is a different proposition again. Each has its own parameters on each tea that supports it, and none of them is a shortcut to another. Choosing between them is a choice about which drink you want rather than about how much time you have, though it is usually made on the second basis.