Bottle nipple size

So my 9 month old has still been on size 2 Dr Brown nipples and that's for age 6 months+. She would take like 10-15 minutes to finish a bottle. I tried to switch her to size 3 (age 9 months+) but then she finishes a bottle in like 5 minutes which I think is too fast. Am I doing her any Injustice if I leave her on size 2?