I had that problem before.
I asked the salesgirl in the store. They said small hole teat is good for drinking water. For drinking milk, medium hole should be used. So I changed all the teats for my bb and now she is drinking milk very well.
I have asked the Avent's agent but she said #1 is designed for BB age 0-3 months, #2 is 3-6 months. But I will try #2 and see whether the eating speed is improved.