While writing in C is one way to speed up R code, you can also get pretty close to compiled speed by writing fully vectorized R code and pre-allocating vectors. The R REPL is just a thin wrapper over a bunch of C functions, and a careful programmer can ensure that allocation and copy operations (the slow bits) are kept to a minimum.
This is good if your code can be expressed in vectorized operations and doesn't gain benefits from problem structure exploited by multiple dispatch. With R the best you can is the speed of someone else's (or your) C code while Julia can beat C.
You’d be surprised how many people don’t know that a for loop isn’t great compared to vectorizing. The same for Julia, few will know that types have an impact on speed. My point is that you won’t automatically write faster code.
