It doesn´t have a simple answer, there are plenty of factors. You can make a search with the key-words "aprender inglês" crawling within this very site/Forum.
Some people are a visual, others are auditory learners, and there are many that learn more with other learning styles.https://en.wikipedia.org/wiki/Learning_styles
To me, the student that learns more and quicker, are the ones that are really "into" the subject, the ones with a degree of passion, they love the subject rather than being motivated by some extraneous motivation (a better payment/a job promotion, for example).
Another important factor in play is having a good teacher/English literature, learning material etc, it boosts the learning in no time.