Questions come up fairly frequently about how to make R code faster. This is an attempt to provide a general framework for thinking about the problem.

Questions of this nature seem to fall into one of a few categories:

• I have a loop and it's running slowly. I've heard that vectorization can speed things up. How do I vectorize it?
• I've vectorized and it's still running slowly. What do I do next?
• I'd like to speed up my code but it's running quickly enough already.

What are the principles and specifics which can be used to make R code run faster?