I faced this problem while interning in the Google Chrome team and working on Chromium. Most of my time was used by reading the source code and figuring out the bigger picture of how classes and methods play together. I actually came to the conclusion that the big problem is that current tooling does not really support a developer on the task of reading existing source code. So I started building a new tool, Coati, an interactive source explorer for C/C++ based on clang. After using Coati myself for about half a year now, I can say that it really helps me navigate through the codebase quicker.
The tool is still at an early stage. We recently launched our website with a first release version: https://www.coati.io/
Do you think Coati is a tool that can help you?