The imports are handled by the preprocessor in C, C++, and Objective C, which creates one large file for the compiler. Every *.m, *.c, *.cpp file will each get all of the imports.
You can compile code on the command line with the -E flag to see the result after all the #imports are added.
Additionally, this question goes into some detail about #include vs #import, so it might give you more insight:
What is the difference between #import and #include in Objective-C?
As you can image, having lots of extra imports slows compilation. Jetbrain's AppCode has a feature that will optimize imports:
http://www.jetbrains.com/objc/features/