Based on this video, most of the McDonald's food are pre-cooked. Now, it explains why the tastes and texture of different items are so consistent at different McDonald's restaurants in the States.
Here is a description for how breakfast eggs prepared from McDonald website:
Round Egg:
A USDA Grade A cracked into an ‘egg ring’ to get that iconic round shape for Egg McMuffin®.
Scrambled Eggs:
Scrambled eggs are made with liquid eggs that are cooked fresh on grills with real butter.
Folded Eggs:
Folded eggs, like the ones on a Bacon, Egg & Cheese Biscuit, are made with liquid eggs that are pre-cooked and folded before being flash frozen by McDonald's suppliers. Once in McDonald's kitchens, they’re prepared on the grill with real butter.
Sausage Burrito Eggs:
McDonald's suppliers pre-cook liquid eggs with ingredients that include sausage, tomatoes, green chilies, onions and seasonings before flash freezing them to help maintain their taste.
