Why is a simple contract so much slower than a check?

bench.rkt

#lang racket

(define (f/raw x)
  #t)

(define (f/checked x)
  (unless (exact-nonnegative-integer? x)
    (error 'f/checked "blah blah"))
  #t)

(define f/chaperoned
  (chaperone-procedure
    f/raw
    (λ (x) (contract exact-nonnegative-integer? x 'pos 'neg))))

;; using contract-out
(module f racket
  (provide
   (contract-out
     [f/from-module (-> exact-nonnegative-integer? any)]))

  (define (f/from-module x) #t))

(require 'f)

(define/contract (f/contracted x)
  (exact-nonnegative-integer? . -> . any)
  #t)

(define count 100000)

(define-syntax-rule (bench func)
  (begin
    (display (object-name func)) (display ": ")
    (time (for ([i (in-range count)])
              (func i)))
    (void)))

(version)
(bench f/raw)
(bench f/checked)
(bench f/contracted)
(bench f/chaperoned)
(bench f/from-module)

#|
Example output:
  "5.3"
                cpu time  real time  gc time
  f/raw:               4          4        0
  f/checked:           6          6        0
  f/contracted:     1166       1197      211

Naively I would expect the simple contract to result in code
not _too_ much more complicated or slower than the manual check.

1. Why not?

2. Could simple contracts be (nearly) as fast as manual checks?
To get the declarative convenince and clarity of contracts, without
so much overhead?

|#

@samth Running the above I get:

f/raw: cpu time: 4 real time: 4 gc time: 0
f/checked: cpu time: 6 real time: 6 gc time: 0
f/contracted: cpu time: 1288 real time: 1327 gc time: 341
f/raw: cpu time: 204 real time: 211 gc time: 66  <-- really f/chaperone, not f/raw
f/from-module: cpu time: 22 real time: 22 gc time: 0

@samth @takikawa
FWIW I took my original gist further: https://gist.github.com/4049108