There is no such thing as learning quickly while being able to go to extreme depths, unless you are a prodigy and genius that can comprehend information to its deepest level from your first glance. Learning anything is a gradual process, sometimes I stay focused on a problem for days and still not able to solve it, but it's a journey, during the times where I'm puzzled for days is the time I learn the most, I go off on the most wondrous tangents, learn about concepts and questions which I tend to pose from these tangential thought processes and eventually I will still not be able to understand/solve my initial question. Whilst at most times, during this stage of the learning process, I would have already figured out my initial problem, however for the rare instances where I am unable to untangle my initial questions, I move on and keep learning, then one day when I return to the material which I was once confused about, I could understand it without any problems at all. The point is, you must take your time if you really want to understand the fundamentals, it could take you weeks or months just to understand a single concept, but it's well worth it in the end.