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) ⇒ 5.200886011123657
(byte-compile 'silly-loop)
⇒ [Compiled code not shown]
(silly-loop 50000000) ⇒ 0.6239290237426758
In this example, the interpreted code required more than 5 seconds to run, whereas the byte-compiled code required less than 1 second. These results are representative, but actual results may vary.