[racket] first vs car ; last vs cdr ; empty? vs null?

On Fri, Mar 07, 2014 at 11:03:57PM -0500, Carl Eastlund wrote:
> The first/rest operations do not use a memoization table.  They test using
> the list? primitive, which is built in and actually has a couple of tag
> bits reserved on every cons object for memoizing its results.  So the
> operation really is quite fast.

I take this to mean there is a bit in the cons object for storing
whether it is a list, and that this bit is set via `list?'.

Why wouldn't the bit be set via `cons'?

Also, when I do:

  $ racket
  Welcome to Racket v6.0.0.2.
  > (let ((ls (make-list 50000000 0)))
    (time (list? ls))                                                                                             
    (time (list? ls))                                                                                             
    (time (list? ls))                                                                                             
    (time (list? ls))                                                                                             
    (time (list? ls)))

  cpu time: 220 real time: 220 gc time: 0
  cpu time: 112 real time: 112 gc time: 0
  cpu time: 60 real time: 58 gc time: 0
  cpu time: 28 real time: 29 gc time: 0
  cpu time: 16 real time: 15 gc time: 0
  #t

Why does the time decrease gradually, rather than the 2nd and subsequent
times being roughly equivalent?

Thanks.

David