A quick and easy way to increase comprehension is to stop occasionally and imagine teaching the material to an imaginary student (Feynman technique). It might help to scribble some quick notes and simple sketches while you’re studying too. I tend to highlight and write notes directly in my books.
If you extract quick notes, you can also memorize them with the help of techniques like memory journeys and peg lists.
Studying before sleeping seems to help with retention too.