David Smith discusses performance improvements upcoming in R 3.4.0:
A “just-in-time” JIT compiler will be included. While the core R packages have been byte-compiled since 2011, and package authors also have the option of btye-compiling the R code they contain, it was tricky for ordinary R users to gain the benefits of byte-compilation for their own code. In 3.4.0, loops in your R scripts and functions you write will be byte-compiled as you use them (“just-in-time”), so you can get improved performance for your R code without taking any additional actions.
Stay tuned for the release.