Shield vs no shield

So we've been using a nipple shield to breast feed because baby has a tongue tie. We've finally gotten to where he will latch to me. He seems like he fills up a lot quicker this way. We use to nurse about 30 minutes with the shield, but now we can be done in 15 minutes. Tried waking him, stimulating his mind to where he will eat. But nope. He's done. Is there a reason why?