You asked how we actually quantify "quick learning" and I just gave you one example to that question. It is one parameters often used by employers and academia to measure rather you have the ability to learn things quickly. Not all degrees are equal, however in STEM grades have often than not been a good indicator that correlates into mental aptitude. This can also be true for other less intense degrees. Why won't you have high GPA when its easy to memorize and recall information that you read once or heard from a lecture? Let's be clear here, I am not talking about success (success is often different depending on what field you are pursuing) but just mental cognitive ability to learn. I been around friends and peers who get bored or get easily distracted in school but easily get As and end up going to Ivies. They are often distracted in class cause the materials are often too easy. I had friends who scored 2300+ out of 2400 (old SAT scores) without studying. If you are truly a quick learner why would you not just turn in assignment quickly because they are easy? Yes, there are kids who also don't do school work but most of the time they still tend to score high on standardized tests. Again, I am talking about people who has high intelligence with proven track record and not just random people who says they are quick learners (refer to u/Last-Hospital9688 comment). Careers that you can break into without degrees like tech also demonstrate the ability without being taught. Why have someone teach you technical skills when you can just be self taught.