[plt-scheme] DrScheme Faster on Powerset than MzScheme - Why?

Kyle Smith wrote:
> I was actually benchmarking DrScheme against another flavor of Scheme, 
> and I thought the results were unfair, since DrScheme is also an IDE.  
> So I compiled to .zo (which didn't alter the times appreciably) and 
> ran this little powerset routine in DrScheme and MzScheme with the 
> following results.
> -------------- THE CODE --------------
> (module powerset mzscheme
>   (provide go)
> (define (power-set s)
>   (if (null? s)
>       (list '())
>       (let ((rest (power-set (cdr s))))
>         (append (map (lambda(x) (cons (car s) x)) rest) rest))))
> (define (go)
>   (collect-garbage)
>   (time
>    (let ([ps (power-set '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 
> 19 20))])
>      'powerset)))
>   )
> -------------- THE RESULTS ------------
> Welcome to DrScheme, version 369.8-svn2mar2007 [3m].
> Language: Textual (MzScheme, includes R5RS).
> > (require powerset)
> > (go)
> cpu time: 469 real time: 468 gc time: 265
> powerset
> > (go)
> cpu time: 469 real time: 469 gc time: 266
> powerset
> -----------------------------------------------------------------------------------
> C:\Documents and Settings\kyle\My Documents\Scheme\math>mzscheme -q 
> --require powerset.scm
> Welcome to MzScheme v369.8 [3m], Copyright (c) 2004-2007 PLT Scheme Inc.
> > (go)
> cpu time: 563 real time: 563 gc time: 405
> powerset
> > (go)
> cpu time: 563 real time: 563 gc time: 376
> powerset
> > (exit)
> -----------------------------------------------------------------------------------
> I was really surprised that DrScheme beat out MzScheme.   Do I have 
> something setup wrong on the command line of MzScheme, as I don't 
> often use the stand-alone interpreter, so I could be missing something?
Drscheme gave me anything in the range of 530-830, I'm not sure why 
there would be such a variation.
drscheme:
cpu time: 588 real time: 586 gc time: 340
powerset
 >
But mzscheme seemed to do much better the 2nd or 3rd time I ran (go):

$ mzscheme -f x.ss
Welcome to MzScheme version 360, Copyright (c) 2004-2006 PLT Scheme Inc.
cpu time: 824 real time: 829 gc time: 596
 > (go)
cpu time: 284 real time: 285 gc time: 100
powerset
 > (go)
cpu time: 296 real time: 298 gc time: 116
powerset
 > (go)
cpu time: 292 real time: 295 gc time: 116
powerset
 > (go)
cpu time: 292 real time: 291 gc time: 116
powerset

I thought because of the JIT, naturally, but if I turn the JIT off I get 
the same results.
$ mzscheme -f x.ss -j
Welcome to MzScheme version 360, Copyright (c) 2004-2006 PLT Scheme Inc.
cpu time: 896 real time: 901 gc time: 592
 > (go)
cpu time: 360 real time: 358 gc time: 104
powerset
 > (go)
cpu time: 376 real time: 376 gc time: 112
powerset
 > (go)
cpu time: 348 real time: 369 gc time: 108
powerset
 >
Although slightly slower than the JIT'ed one its still much faster than 
drscheme.