For some people, it is normal to preheat the oven before something is put in. However, if you think about it, it is a waste of energy and time because the oven is already generating heat while it warms to its target temperature. Why not use that heat to start cooking sooner? It does not have an impact on most food and gets them cooked faster.
Go ahead and try putting whatever you are baking in the oven right away and then turn the heat on. You can easily save 10% of the heating time, if not more.