Load level / prefabs without causing hiccups

How can i load big prefabs or additive levels without causing game to freeze. I have a many scenes additively loading, making a large map. But as levels load the game freezes. can i do anything to prevent this? Is it possible to sacrifice loading speed so this doesnt happen?

I have unity pro. i am also using lod groups and occulsion culling, but frame rate is around 20-30. and im not done with the map.

For prefabs, have you considered creating an object pool? This might help. The lag could also be because of your computer, what are your specs?