But in terms of code quality within R itself, I was surprised to find that a number of its .c files are actually machine-translated Fortran, so I'm guessing the author's statistics are not far off.
I discovered this when I decided to confirm my suspicions that my (the?) most frequently used function, t(), which computes the transpose of the matrix, was implemented about as naively as possible. If R developers were really concerned with speed, this is probably the first place to start optimizing.