Whether food cooks faster with the lid on or off depends on the food, its size, the container and the oven temperature. Some foods simply can't be covered during cooking, while others offer some flexibility. Base your cooking method on what's best for a specific dish, rather than speed of preparation and you'll get satisfactory results every time.
Casseroles with creamy fillings, such as broccoli, cheese and rice, lasagna, or macaroni and cheese should be covered initially while the ingredients heat up. Covering them helps them cook faster and also keeps them moist by holding in steam. You can also use this approach with breaded meats and stews. Once the dish is bubbling and hot, remove the lid if you want to brown the top. Otherwise, you can leave the lid on from start to finish.
In almost every case, you'll want to bake cookies, bread and other baked goods without a lid. The reason is that these foods need to rise, or expand, in the oven, and they need to brown. A lid would prevent both actions. For best baking, use shiny aluminum pans, rather than glass, and set the oven at 350 to 375 degrees Fahrenheit. A convection oven cooks baked goods about 30 percent faster and creates an overall golden finish. Baked goods come out tender on the inside and crisp on the outside.
Most types of oven cooking, such as roasting and baking, involve dry heat. By placing a lid over the food, you create moist heat. Any time you roast a food, leave the lid off. Roasting meats and vegetables cooks them quickly, while creating a golden crust and tender interior. Place meat in a roasting pan and place vegetables on a baking sheet. Brush with oil to help the food brown and roast at 375 to 400 F.
How you cook the food can influence how you prepare and season it. For example, if you were roasting a chicken, you might brush it with oil and season it, but you wouldn't add liquid. If, on the other hand, you want to cover chicken and poach it in the oven, you'd add chicken broth or wine to keep the food moist. In almost every case, covered foods perform best when cooked with a bit of moisture. Roasting tends to intensify flavors, while braising, steaming or poaching tend to dilute flavors. You might need to adjust flavorings depending on your approach.