The whole purpose of lightmaps and static lightning building is to save computing power when the game runs. I believe lighting calculation and AA are the most expensive operations so the FPS drop from 30 to 9-15 seems possible.
You can try to lower lightmaps resolution to increase the build speed.
You should be able to use level streaming so you can split the whole world into smaller levels that will be loaded/unloaded seamlesly as player moves near/far and you should be able to build lighting always only for the level you are currently working on instead of all the world.
To improve dynamic lighting performance you can also try to limit lights that cast shadows (these are more expensive).
Last but not least check the global Lightmass section under World Settings; there are many options that can affect both static and dynamic lightning performance greatly.