The Build Graph parses your Derived Data folder to find out build logs. 

You can open any project, and see visualisation of a build:
- Order of modules;
- Compile time of each module;
- Dependencies between modules;
- Places of low concurrency which can be parallelised to speed up whole build;
- Dive into module's build to understand what has influence on build's duration;
- Filter any kind of task to simplify graph.

You'll get the best result after clean build of multi-module app.