iPad Air wifi sync a lot slower than iPhone 5s?

I just got my new iPhone 5s yesterday and found out that its wifi syncing to itunes is a lot faster than the iPad Air I bought a few days ago. Actually iPad Air wifi syncing is so much slower than iPhone 5s, almost half speed, just as slow as wifi syncing with my iPhone 4, is this normal?

I've tested another iPad Air of my friend, and found out that his new iPad is as slow as mine. Anyone know WHY? Is there anything / settings we're missing?

I have the latest AirPort Extreme as wifi router.