All the cooking times in Star Chef are set with increasing difficulty in mind - the higher you level up the harder the restaurant tasks get.
When you first start the game, the items such as Cream of Mushroom Soup and Roasted Tomato Soup have lower cooking times to allow all the new players to progress at a reasonable pace. As you progress in the game, the more difficult the orders become and the bigger the rewards get.
Each time you gain a level, the amount of experience needed to attain the next level will rise. By adjusting the cooking time and the rewards you gain for each dish, the game remains balanced and fun for everyone.
Of course there is a choice for everyone to use cash to speed up the process; so if you are a rush, go ahead and try instant cooking!