(define a '((vx . 123)
            (vy . 84)
            (vz . -39)
            (z . 281)
            (y . 384)
            (x . 38)))

(define undefined '(undefined))
(define (undefined? o) (eqv? o undefined))


(define (myassoc searched-key alist)
  (if (null? alist)
      undefined
      (let* ((binding (car alist)))
        (if (eqv? (car binding) searched-key)
            (cdr binding)
            (myassoc searched-key (cdr alist))))))

(define (test-vals)
  (let loop ((alist a)
             (x undefined)
             (y undefined)
             (z undefined))
    (if (or (null? alist)
            (not (or (undefined? x)
                     (undefined? y)
                     (undefined? z))))
        (void x y z)
        (let* ((binding (car alist))
               (rest (cdr alist))
               (key (car binding))
               (value (cdr binding)))
          (cond ((and (eqv? key 'x) (undefined? x))
                 (loop rest value y z))
                ((and (eqv? key 'y) (undefined? y))
                 (loop rest x value z))
                ((and (eqv? key 'z) (undefined? z))
                 (loop rest x y value))
                (else
                 (loop rest x y z)))))))

(define (test-naive)
  (let ((x (myassoc 'x a))
	(y (myassoc 'y a))
	(z (myassoc 'z a)))
    (void x y z)))

(define (bench times)
  (print "bench test-naive")
  (time
   (let loop ((i 0))
     (test-naive)
     (unless (= i times)
       (loop (add1 i)))))

  (print "bench test-vals")
  (time
   (let loop ((i 0))
     (test-vals)
     (unless (= i times)
       (loop (add1 i))))))

(cond-expand
  ((or compiling script) (bench 10000000))
  (else (void)))


Results:

[kooda@black-star /tmp] % csc test.scm    
[kooda@black-star /tmp] % ./test
bench test-naive
16.111s CPU time, 0.265s GC time (major), 176/43408 GCs (major/minor)
bench test-vals
21.364s CPU time, 1.754s GC time (major), 1206/134823 GCs (major/minor)


[kooda@black-star /tmp] % csc -O5 test.scm           
[kooda@black-star /tmp] % ./test
bench test-naive
2.867s CPU time, 0/1221 GCs (major/minor)
bench test-vals
8.559s CPU time, 0.811s GC time (major), 557/88112 GCs (major/minor)