Q:

Are there researched approaches to learning a subject more quickly?

TLDR at bottom.
So from my knowledge(correct me if I'm wrong) there is no evidence to suggest that we can improve general intelligence. But what about improving skills/intuition in specialized fields like programming or mathematics? 
I went through this course on coursera called Learning how to learn by Barbara Oakley(engineering professor) who suggests the following to quickly learn new subjects: 

Pomodoro technique 25 minutes study 5 min break
Eliminate Distractions
Do not neglect memory training. Recalling what you have studied is a great way to learn.
Practice and consistency. 
Cramming is bad - it takes time to 'build neural chunks' as she says. I wasn't sure if this was based off of actual science or just conceptual. 

I do not like method 1 at all from my experience. Often times when I am going through technical textbooks and I have a timer randomly go off on me at 25 minutes in the session, I find myself jumping from my seat and my productivity wanes. However, timers at around 45 minutes(where my brain seems to slow down) does not have this kind of effect.
But in Cal Newports Deep Work book, he suggests:

Absolutely no distractions. Even delete social media from your life. Isolation can be a very ideal way of achieving deep levels of focus 
Be consistent.
Do not tap in to willpower
Use 'rituals' to turn the deep work mode on. 

From my experience, point 1 has severely reduced my ADHD symptoms and as a result my productivity has greatly improved. But even then, I still wonder to myself how much validity there are to these two professors approaches? Neither are neuroscientists, cognitive scientists, or psychology professors. I want to know if there is some kind of research, evidence(strong or weak), that there are more effective approaches to learning a specific field at a much faster pace. I have also read summaries of 'Peak' by Anders Erikson who is an apparent authority in this topic but I still haven't found anything eye opening. 
Basically, is it possible to learn a subject at a much faster pace through the use of different studying techniques such as mnemonics, spaced repetition, etc.

A:

This question is pretty vague and broad, but the most researched techniques, according to a reasonably recent (and highly cited) review, Dunlosky et al. (2013):

It looks like distributive practice (spreading out activities over time, the opposite of cramming, I guess) and practice testing (i.e. trying to apply what you've learned) get the highest generalizability ranking. The techniques that get a moderate rating are somewhat related to these two (interleaved practice: alternating the types of activities involved), self-explanation (being aware of the steps during problem solving) and elaborative interrogation (somewhat related to the previous one: explaining [oneself] why some statements are true based on the previously learned concepts). Mnemonics, mental imagery, summarization, highlighting or re-reading were ranked low in overall utility. 
But even for the high utility techniques some questions remain (that's why they're tagged with P-I for instance):

For instance, self-explanation
  received a P-I rating for criterion tasks because the
  available evidence is positive on one dimension (generalizability
  across a range of criterion tasks) but is insufficient on
  another key dimension (whether the benefit of self-explanation
  generalizes across longer retention intervals). As another
  example, rereading received a Q-I rating for criterion tasks
  because evidence for the effectiveness of this technique over
  long retention intervals is qualified (i.e., under some learning
  conditions, it does not produce an effect for longer retention
  intervals), and insufficient evidence is available that is relevant
  to its effectiveness across different kinds of criterion
  tasks (e.g., rereading does boost performance on recall tasks,
  but little is known as to its benefits for comprehension). 

You'll have to read the full for details as it's too much material to summarize here.
Also, I can't say if the findings still hold for special groups like ADHD.

