you could try to enlarge the hole or punch another hole on the teat. It works for me. He used to have the same problem but now he could now finish his milk (6oz) within 15 min. As the baby drinks more now and he is not under the spur period, it takes him longer to finish his milk and he will be very tired if keep sucking for a long time.
My bb is now 2 months old & he seems to drink less milk. He used to drink 7 oz quickly, now he will left 80-100ml due to sleeping. No way to wake him up, it takes almost an hour and still have milk leftover.