Boiling water is at 100°C / 212°F, and an oven is often set to 180°C / 355°F; even so, a dish cooked in water heats faster than one in the oven. The reason is that water carries heat much faster than air. Air is less dense and transfers much less heat to the surface of food in a given amount of time. That is why water and air at the same temperature are two very different cooking mediums in the kitchen.
The difference comes from how much matter is in the same volume. Water is a dense liquid; each drop that touches the surface of the food carries heat, and as it cools, another drop immediately takes its place. Air is less dense; the same volume contains much less matter and therefore carries much less heat. That is why 100°C / 212°F water heats food many times faster than 100°C / 212°F air. The fact that you cannot keep your hand in boiling water for one second, but can briefly reach into a 180°C / 355°F oven, is everyday evidence of the same difference. Still, never reach into a hot oven or touch hot pans or racks with bare hands; use oven mitts.
Heat transfer changes when steam is involved. Steam releases heat as it condenses on a surface, and contact with steam can cause serious burns. Hot air or steam escaping when an oven door opens can burn the face, so keep your face away from the oven when opening the door.
Oil falls between water and air. Its density is close to water’s and it carries heat well; moreover, because it does not boil at 100°C / 212°F, it can reach much higher temperatures. That is why oil at 170–190°C / 340–375°F cooks much faster than oven air at the same temperature and browns the surface in a short time. The same potato browns in half an hour in the oven and in a few minutes in hot oil; the difference comes from the medium carrying the heat, not from the temperature.
The practical takeaway is to choose the method. Use water or steam when you want the inside to cook quickly and evenly; use an oven or oil when you want the surface to dry and brown. Food cooked in water does not brown because its surface cannot rise above 100°C / 212°F, and the reactions that cause browning do not occur at that temperature. Food cooked in an oven can develop color and a crust because its surface can dry out and heat up. Parboiling thick vegetables before roasting combines these two advantages. The same principle applies to cooking fries at a lower temperature first and then frying them at a higher temperature.
Remember this difference when comparing cooking times, too. A vegetable that a recipe says to cook “for 10 minutes in boiling water” takes much longer to reach the same softness in the oven. Methods such as sous vide are based on the same principle: 60°C / 140°F water heats much more effectively than 60°C / 140°F air. That is why even a change of a few °C in the temperature of food cooked in water can immediately change the result. Dishes cooked in a water bath in the oven also benefit from this medium, which carries heat gently but quickly, and it reduces the problem of a dish that cooks around the edges while remaining raw in the center.