Very often I need to explain difference between comprehension and retention. An interesting tool to explain this is Bloom’s taxonomy of cognitive levels. While there is controversy related to validity of the taxonomy and which variation of the taxonomy is correct, I will use it below as a visualization tool.
For more detailed explanation of each step please see e.g. this page.
As you can see there are several cognitive processes associated with learning which are different from each other.
If a person can memorize something, this means that the person can recreate and identify the piece of knowledge memorized. However, additional levels of cognitive processing are required to manipulate the knowledge: interpret it in another context, apply it when facing new challenges, compare it with other concepts, modify it into new contexts, judge its effects. To do all of these operations we need to be able to divide the idea into its components and understand how his components connect with each other, additionally we need to connect the whole idea and the relevant components to other ideas we have acquired.
For example you are about to learn a formula. A simple way to do it is use Major Method memorization, memorize all the letters and math symbols in the formula. Then each time we need to recreate the formula, it is readily there. Does it mean we will be able to use the formula to solve physics exam or to build a functioning mechanism? Probably not. Even worse will happen if someone changes the notation. We probably need some other way to remember formulas. To do that we can divide the formula into elements and understand how each element influences the result. Alternatively we can try to understand how to derive the formula, then we will be able to modify the formula when the underlying assumptions no longer hold.
Another example is definitions. Students often learn definitions word by word. Then somebody changes the underlying assumptions or provides a different definition. Is this different definition right or is it wrong? It is better if the student can derive the definition in the first place, or can understand the effect of each phrase within the definition, then it is easier to find similarities and contradictions in alternative definition.
You can read an article, remember it word by word and not to learn anything useful. You can read an article, learn something useful you will alter utilize every day, but then forget where you read it and how it is called. Proper learning experience assumes than comprehension and retention are present together and all the time, otherwise how are we going to build additional layers of knowledge on top of that?
After saying what I just said, you would expect me to present a mind-blowing toolset that will solve all the problems. It is very easy to improve both, see e.g. here or here. However there is not very much I can currently offer to speed up higher cognitive functions. They will take time. At some point, reading may become slow again. Not because you will be easy reading or trying to remember, but because you will be busy trying to solve higher level of questions: how to derive argumentation, analyse assumptions and results, modify and apply the solution to existing challenges.