You didn't answer our questions, so I'll give a rather general answer:
For very young or premature babies, nursing can be quite exhausting. This may mean that they will actively "drink" for a short while, then take a small break ( nipple in mouth, sucking or not), then start over. Sometimes, they will even fall asleep during the meal.
For breastfed babies, this matches the maternal let-down reflex (the milk flow), which comes in "waves". Many babies will "use" the reduced flow for a break, then start sucking harder and thus trigger a new milk wave.
Remember, not all babies are created equal: My firstborn took "forever" to drink (up to 45 minutes, but then he drank "until the cup floweth over"), my second was usually done in minutes (we dubbed her "the hoover").
So what would be my advice?
Relax. This is perhaps the most valuable piece of advice one can give new parents. I can't give you medical advice, but I will tell you, what I'd do (and did):
- Take our time. If my baby needs 35 minutes, why take the bottle or the breast away after 30? Many pediatricians have a tendency to "interfere". Nursing time is a great time to relax together, to bond and to cuddle. Especially for the recuperating mother it ensures the necessary breaks. And find something to do, if necessary: read, make a phone call, close your eyes after a short night...
- Roughly (!) monitor growth and weight gain. If baby is doing fine, she is not burning too many calories. She might be hungry sooner, she might be a bit more tired, because she took so long to nurse, but so what?
- If she's not developing well, then I'd find a way to get more calories into her or talk to our pediatrician again.