Retention is the worst way to measure understanding.
Whenever you are reading a new concept, do not optimize to remember and retain it word by word; instead, focus on understanding the underlying pattern of the problem and how the approach solved it.
This is true when you are learning how systems are designed, scaled, and implemented; or working with data structures, algorithms, foundation computer science concepts; or even learning new frameworks and programming languages.
Given the age we live in, where everything is searchable at our fingertips, the problem of expanding and looking up is non-existent. So, focus and put your energy into understanding things really well, and it will reflect well in your career trajectory.