[racket] 2htdp/image Sierpinski

In 2htdp/image the implementations of Sierpinski triangle and carpet are nice and simple, which is great for using them in class.


(define (carpet size c)
  (if (<= size 2)
      (square size "outline" c)
      (local [(define sub (carpet (/ size 3) c))
              (define ctr (square (/ size 3) "solid" "white"))]
        (overlay (square size "outline" c)
                 (above (beside sub sub sub)
                        (beside sub ctr sub)
                        (beside sub sub sub))))))

But...  (carpet 200 "red") produces some strange behavior on my Mac. (carpet 300 "red") is even worse.  The image draws very slowly, which I assume comes from the way the image is actually composed of all the composed images rather than a single bitmap. But, stranger than that, it seems to need to draw itself several times. 

Also I would have thought that 'forcing' the image by actually drawing it would save the bitmap so that future drawing was faster. But I may not understand how this is actually working.

Is this style of using above, beside and friends to make something this big intended to work? Or is there a different style I should be using?