The key to understanding something is to know the what, why, how, where, when of a subject. Fast learners do these connections automatically to some extent. You can artificially improve it by using techniques like mind-mapping.
A lot of topics need muscle memory. If you can know multiplication tables, physics constants, some conversion ratios, you can easily do many calculations in your mind while reading and assimilate the subject better.
Muscle memory is also useful in deducing solutions/solution patterns for common problem patterns.
Every subject has some fundamental concepts, constants, rules, exceptions and patterns you should know at your fingertips. This will definitely speed up of understanding
I have seen people speed read naturally. Fortunately, you can gain the skill too, if you are conscious about speed while reading.
It is most effective in novels and least effective in math. Moderately effective in narrative subjects. The key idea is not to read by mouth but just glance at the content and read important words. Skipping few words is fine as long as you are able to understand the flow.
Very effective in revision (See a point on revision)
Fast learners inherently look for summary and reconstruct everything else as needed. Some books have summary section at the end of the chapter which gives main points. Most fast learners automatically skip or speed read less important stuff.
Early bird expert:
If you are early bird reader on a subject, everybody comes to you for their doubts. This is very good because, you understand the subject more by teaching and you understand the subject deeper by solving other’s doubts. In turn other think that you are a fast learner whereas you are learning by repetition and discussion. This is the best technique because, all you need to do is read and understand a few days earlier and everyone else will make you an expert.
Patterns are easy to remember and understand than pure concepts. Patterns can exist at multiple levels.
Some have intuition based on the way and environment they grow. This intuition helps in assimilating and connecting the knowledge the person is reading. Intuition also develops when you understand a lot of topics and you can extract patterns across.
When we are curious of something, our mind automatically applies many of the techniques listed in this thread! Yes, your mind already knows a lot of techniques.
The more sensory organs you involve the more your brain is able associate. For example a child learns objects better when he sees, hears, smells and feels the objects than just reading it on paper.
Experience and exposure:
A lot of people have prior experience or exposure to content before actually formally learning the subject. For eg: a Software Engineer’s son is already exposed to a lot of concepts before actually taking the subject. He might not understand everything when he is exposed to that knowledge but He will be able to connect the dots when he formally learns and think beyond what he is studying.
Association is the best memory technique. People who can associate new learning with what they already know can remember better and move faster.
Order of learning:
Order of reading subjects also makes a huge difference. For example if you learn music informally first, you will be able to learn and correlate music theory concepts. It wont be as effective if you try the other way.
You might have noticed that quick learners also get doubts at random time. They are involuntarily replaying the material in their brains. Replaying serves three purposes: 1. improves retention 2. Makes you realize different perspectives of the same content 3. think in dimensions you missed while reading the first time.
State of mind:
Most people with less distractions/tensions in life will be able to put more of their mind time on the subject for the same amount of physical time you spend in front of the books. Our minds constantly wander on all different topics. It is the nature of mind. The lesser things you have to wander the more time you can spend on the content you are interested in.
Some people on the thread have mentioned it in a different way “Study when you are happy…”. The fundamental principle is how much of your mind time is occupied.
First exposure on the subject:
First book and/or the first teacher you encounter has a great influence. If you have a teacher with great presentation skills or a book with simple to understand illustrations, then your effort is reduced by 50% . Any further books you read, you can easily assimilate extra information quickly. Choose your first teacher/book wisely.
When foundation is strong, anything learnt on top of it is faster. For eg: if you know the concept of “speed” you can understand the concept of “acceleration” faster. Also if you understand the concept of “rate of change” in math then you can understand both concepts “speed” and “acceleration” almost immediately. Strong understanding at lower levels results in faster understanding at higher levels. Similarly, core subject expertise helps in faster learning of related subjects.
Learning based games help you speed up ! Several online games and mobile apps are focused on learning in innovative ways. Make use of them. It is fun and useful at the same time.
Some other good resources:
1. The first 20 hours: How to learn anything: Key idea is to getting your hands dirty as quickly as possible and learn on the field.
2. How teaching helps you study better
Ramakrishna Rajanna’s answer to What is the best study method?
3. Feynman technique
Acaz Pereira’s answer to Learning New Things: How can you learn faster?
Most important: I have seen people who consistently work hard outpace the quick-learners sooner or later in life.