I imagine your mileage may vary depending on the functions and operations that you are performing. I did a simple test of python vs R and python was the overwhelmingly faster. I believe both languages are primarily written in C, and as such, the speed of your program will depend on the 'efficiency' of the underlying code for the functions you are using.
R is horrifically slow on loops. If you can vectorize the operation, then it will be reasonably fast. If you have to use an explicit loop, then python will be faster. My previous company had a saying that "R is fast if your write it in C". If you have many nested loops, you'll probably need to write a C extension.
