A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:
(defun silly-loop (n) "Return the time, in seconds, to run N iterations of a loop." (let ((t1 (float-time))) (while (> (setq n (1- n)) 0)) (- (float-time) t1))) ⇒ silly-loop
(silly-loop 50000000) ⇒ 10.235304117202759
(byte-compile 'silly-loop) ⇒ [Compiled code not shown]
(silly-loop 50000000) ⇒ 3.705854892730713
In this example, the interpreted code required 10 seconds to run, whereas the byte-compiled code required less than 4 seconds. These results are representative, but actual results may vary.