There are a couple of long-winded articles around about how to do this specific technique, however I have not seen anything quick and to the point.

The first thing you are going to want to do is turn on Whole Module Optimization in Debug (by default, it is only on in Release).

Here are the optimization settings:

Before:



After:



The next thing you want to do is specify -Onone in Other Swift Flags in Debug like so:



This will allow Whole Module Optimization compile the whole module at once while also not doing any optimization of the code.

By using this technique, I have seen gains of up to 5x.

Try this out and let me know how it works for you!