Matsumoto Yukihiro (Matz) is the author of the programming language Ruby. He says about how he read the source codes:

Do not try to read the whole codes.

There is no "story" in the source code, so you do not need to read through it all. It is enough if you pinch interesting fragments and learn the wisdom of the predecessors.

Read with purpose.

If you read the source code to learn some particular thing, you can effectively understand and acquire knowledge.

In other words, you should better to have a particular purpose, and cherry-pick interesting fragments from the codes.

I introduced the three ideas of lazy evaluation study method, YAGNI principle and Matz's code reading method. You may find common patterns among the three ideas.