Swift incremental builds take awhile and I can't tell what is taking up so much of the time

Here's a picture showing my issue:

Essentially I'm changing a large Swift file, say a number within a function from 10.0 to 12.0 (a small change) and incrementally compiling. When Building with Timing Summary (see screenshot above) I can see that it takes about 3 seconds for Swift to compile the changes in the file and another 2 seconds to seemingly merge them in.

But where I'm confused is then it sits for another ~14 seconds opaquely (3x longer than the "compile") and then updates the estimate to 19 seconds but it's unclear where the added time came from since it doesn't show anything. What is it doing during this step? Is there a way I can find out so I can potentially optimize it?

All Xcode build settings are set to the defaults including the Build System, using Xcode 11.2.1 on Mojave.