I don't think there is a much difference if you're not processing data on the fly. You'd find both the languages doing fair on that point. Like say Python can process both server side and desktop side data pretty quickly.
R too have shown it's performance on offline data. However I am skeptical on performance of R language on the server side. e.g. Reading and saving data files in R often have different performance speed for Python and R. I have found server side, python performing better.
Depending on how quickly you want to process the data, it'd be different for both languages. My preference in such case is for Python.