Improve function efficiency

[Brian M. Schilder]

Some key ways to generally improve the efficiency of your code:

• Use lapply or Reduce instead of for loops where possible.
• Use data.tables instead of data.frames and take advantage of their speedy vectorized transformations.
• Consider parallelizing steps across multiple cores for speed boosts (e.g. via the parallel package). I'll leave it to you to decide whether (and where) you'd like to implement this.