In academia, does the amount of material that one has to learn diminish as time progresses?
The first paper I read took two months to process.
Now, I can skim through two papers for breakfast. It is not that you get more material to read, but rather you get much more efficient at skipping things you know or recognize as unimportant. 

It comes with practice - try reading papers and books, and think about what are the important parts. Learn to identify the 'meat' and which techniques are used. Also, you'll notice that instead of learning 'the stuff', it is about cataloging and storing meta-information about where to find 'the stuff' once you really need it.

After a while, you realize that most of the new papers you read, you only need the gist of it, in order to reference it. Comparably few papers need to be read and understood in paragraph-by-paragraph detail.