15 minutes isn't caramelizing them though, it's browning them. It's the same reason you can't cook a cake at 450° for a shorter time when directions are 350°
You might bring them to the same end temperature, but there are reactions occurring which take time in both examples that higher heat will not make happen faster.
A French onion soup with 15 minute caramelized onions would not be good
15 minutes isn't caramelizing them though, it's browning them. It's the same reason you can't cook a cake at 450° for a shorter time when directions are 350°
You might bring them to the same end temperature, but there are reactions occurring which take time in both examples that higher heat will not make happen faster.
A French onion soup with 15 minute caramelized onions would not be good