(declaim (optimize  (compilation-speed 0)))

(defun my5 ()
  (declare (optimize (speed 3) (space 0) (safety 0) (debug 0)))
  (loop for num from  1
     until (loop for divisor from 20 downto 1
          always (zerop (mod (the fixnum num)
                 (the fixnum divisor))))
     finally (return num)))

Why do I get:
 note: doing signed word to integer coercion (cost 20) to NUM

; in: DEFUN MY5
;     (LOOP FOR NUM FROM (THE WORD 1)
;           UNTIL (LOOP FOR DIVISOR FROM 20 DOWNTO 1
;                       ALWAYS (ZEROP (MOD (THE FIXNUM NUM) (THE FIXNUM DIVISOR))))
;           FINALLY (RETURN NUM))
; --> BLOCK LET SB-LOOP::LOOP-BODY TAGBODY SB-LOOP::LOOP-REALLY-DESETQ
; ==>
;   (SETQ NUM (1+ NUM))
;
; note: w
;
; compilation unit finished
;   printed 1 note