Consider two identical 4-kg pieces of roast beef. the first piece is baked as a whole, while the second is baked after being cut into two equal pieces in the same oven. why is there a difference between the cooking times of the whole and cut roasts?

Too cook the beef, the heat should penetrate to the deepest layer of the meat. Let say that the meat is cubical with 10cm width. To penetrate the core of the meat, the heat need to travel 5 cm from any side of the meat. If the meat cut into two similar pieces, the deepest core of the meat will be exposed to the heat directly.

Now the deepest core is not 5 cm anymore, it will become 2.5 cm. So the heat has to penetrate a thinner layer, thus make it cooked faster.