reading a file into a string on ACL

Greetings, 

When I try to read a file into an array using ACL it is painfully
slow. The function I use is this:

(defun file->string (file-name)
  "Reads file <file-name> returning a string of its contents."
  (proclaim '(optimize (speed 3) (safety 0)))
  (let ((result (make-array 0 :adjustable t :fill-pointer 0)))
    (with-open-file (stream file-name :direction :input)
      (do ((char (read-char stream nil nil)
                 (read-char stream nil nil)))
          ((null char))
        (vector-push-extend char result)))
    (coerce result 'string)))

When I run the function on a file of 442K it takes close to 40 minutes
to run. 

cl-user(33): (let ((*print-pretty* nil)) 
  (time (progn (file->string "tmp1.txt") (values))))
8727128 bytes have been tenured, next gc will be global.
See the documentation for variable *global-gc-behavior* for more
information.
; cpu time (non-gc) 1,767,970 msec (00:29:27.970) user, 2,240 msec
system
; cpu time (gc)     162,860 msec (00:02:42.860) user, 440 msec system
; cpu time (total)  1,930,830 msec (00:32:10.830) user, 2,680 msec
system
; real time  2,268,903 msec (00:37:48.903)
; space allocation:
;  135,774 cons cells, -1,008,972,256 other bytes, 0 static bytes
cl-user(34):

When I run it on CMUCL, it takes about 0.2 seconds.  

USER> (let ((*print-pretty* nil)) (time (progn (file->string "tmp1.txt")
(values))))
; Compiling LAMBDA NIL: 
; Compiling Top-Level Form: 

; Evaluation took:
;   0.23 seconds of real time
;   0.2 seconds of user run time
;   0.0 seconds of system run time
;   461,131,944 CPU cycles
;   1 page fault and
;   4,653,584 bytes consed.
; 
USER> 

So this trial takes about 10 000 times as long on CMUCL as on ACL. 

I'm using ACL 6.2 on Redhat 9 x86. Does anyone have any idea what is
going on here? Could it be a bug? Or am I missing something here? 

Thanks,

martin