Scene generation question

I am working on a game where one of the key game play mechanics is that the map the game takes place on is always different. All maps are basically outdoor towns and cities and the important characteristics that change will be what buildings are present and where they are located. To accomplish this I'm making prefabs for different types of buildings and then using instantiate to place them on the map in a random yet algorithmically deduced way. My problem is that when I test a map with more than 9 buildings there is a huge amount of lag for the first few minutes while the game instantiates the buildings. Is there a way to listen for a "on complete" type of event? Or is there perhaps a better way to accomplish the effect I am going for. I am new to unity and worried that this method will rule out any baking of light maps and other things done to improve performance. Any advice would be appreciated.