Are there researched approaches to learning a subject more quickly?
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.