With time and experience, reading code becomes much easier.

And well-written code is usually easy to read and understand too!

The purpose of a code review is, apart from ensuring correctness, to ensure that the code that gets merged is easy to understand! And to be honest, if it's easy to understand, it's easy to ensure correctness too!

The biggest challenge I had was to distinguish between explanations needed to understand the change, and explanations needed to understand the code after it was merged in. And making it clear in my code review questions that whatever question I have, I need code and comments in the code to answer them, not the author to explain it to me (I frequently have already figured out the why, but took me longer than needed): it's not because I did not get it, it's because it should be clearer (finding the right balance between asking explicitly, offering a suggestion, or pitting it as a question to prompt some thinking is non-trivial too).