Can it save build time when `N configs (1 build) -> N packages`?

As the do said in https://github.com/conan-io/docs/blob/master/creating_packages/package_approaches.rst:
```
So if we just have Debug/Release build types, and we’re producing N packages for N different configurations, we’ll have N/2 build folders, saving half of the build time.
```
Debug and Release should be built respectively, how can it save half of the build time?

Read creating_packages/package_approaches.rst, especially the passage about Debug/Release configurations and N packages. Clarify how the stated build-time saving works, then update the documentation so the explanation is unambiguous.

Written by the indexing model from the issue text.