Is R a compiled language?
In most cases R is an interpreted language that runs in a read-evaluate-print loop. There are numerous extensions to R that are written in other languages like C and Fortran where speed or interfacing with native libraries is helpful.
I've often rewritten R code in C++ and made it run 100x faster. Looping is especially inefficient in R.
R is generally an interpreted language. However, package compiler offers bytecode compilation that can improve performance. You can also call compiled code from R.
In terms of how fast, it depends on what you are trying to do and how you are trying to do it. Some looping operations can be very slow. However, in many cases, with well written code, the performance of R scripting is determined by the speed of the underlying internal C-based libraries and system memory read-write speeds, and so R is about as fast as anything else.