You're not wrong and absolutely totally wrong at the same time. R is the furthest thing from C you could find in paradigm, syntax and performance, but yes much of the underlying code is C or Fortran.

But really you're missing the point. R's purpose is interactive, exploratory and scientific computing and that's what it is incredibly good at. It wasn't intended for high performance computing, but there are ways of getting it there. Look out for Rho in the future.