efficient memoize decorator?

[thattommyhallll@gmail.com]

im plugging away at the problems at
http://www.mathschallenge.net/index.php?section=project
im trying to use them as a motivator to get into advanced topics in
python.
one thing that Structure And Interpretation Of Computer Programs
teaches is that memoisation is good.
all of the memoize decorators at the python cookbook seem to make my
code slower.
is using a decorator a lazy and inefficient way of doing memoization?
can anyone point me to where would explain how to do it quickly. or is
my function at fault?

the code in question is as follows
"""
from memoize import memoize,memoize2

@memoize
def col(n, count):
    if n == 1:
        return count
    elif n % 2 == 0:
        return col(n/2, count+1)
    else:
        return col((3*n+1)/2, count+2)

import psyco
psyco.full()

start = time()
maxlength = 0
best = 0
for i in range(1, 1000001):
    length = col(i,1)
    if length > maxlength:
        maxlength = length
        best = i
        print maxlength, best
end = time()
print 'took', end-start