[plt-scheme] Strange benchmark for string=?

 > (let ((s1 (make-string 50000 #\a))
         (s2 (make-string 50000 #\a)))
     (time (do ((i 0 (+ i 1)))
             ((= i 10000))
             (string=? s1 s2))))
cpu time: 1902 real time: 2133 gc time: 0

 > (let ((s1 (make-string 50000 #\a))
         (s2 (make-string 50001 #\a)))
     (time (do ((i 0 (+ i 1)))
             ((= i 10000))
             (string=? s1 s2))))
cpu time: 1943 real time: 2604 gc time: 0

 > (let ((s1 (make-string 50000 #\a))
         (s2 (make-string 50000 #\a)))
     (define (my-string=? s1 s2)
       (and (= (string-length s1)
               (string-length s2))
            (string=? s1 s2)))
     (time (do ((i 0 (+ i 1)))
             ((= i 10000))
             (my-string=? s1 s2))))
cpu time: 1883 real time: 2012 gc time: 0

 > (let ((s1 (make-string 50000 #\a))
         (s2 (make-string 50001 #\a)))
     (define (my-string=? s1 s2)
       (and (= (string-length s1)
               (string-length s2))
            (string=? s1 s2)))
     (time (do ((i 0 (+ i 1)))
             ((= i 10000))
             (my-string=? s1 s2))))
cpu time: 40 real time: 90 gc time: 0


Is this the fault of unicode or just an oversight
in string=? ?

-- 
Jens Axel Søgaard