Clojure 1.2.1 vs 1.3.0-beta2 Question

gistfile1.clj

;; This takes about 1.7 seconds in Clojure 1.2.1 and takes 10 times as long in 1.3.0-beta2
;; Also, I'm learning Clojure so ignore the generally horrible code.
;; Tried switching to lazy-seqs prime to see if that narrowed the gap. It's a bit slower, but the gap is still there!
;; Clojure 1.2.1
(time (solve21))
"Elapsed time: 2333.739 msecs"
31626

;; Clojure 1.3.0-beta2
(time (solve21))
"Elapsed time: 17352.799 msecs"
31626

(defn amicable? [a]
  "Returns true if given numbers divisors produce an amicable number"
  (let [b (sum-divisors a)]
    (and (not= a b) (= a (sum-divisors b)))))

(defn amicablepairs [limit]
  "Returns list of all amicable pairs up to given limit"
 (filter amicable? (range 2 limit)))

(defn solve21 []
  (reduce + (amicablepairs 10000)))

;; Was told the old primes-to function could be to blame, switching to clojure.contrib.lazy-seqs prime generator

(defn lazy-primes-to [n]
  (take-while #(< % n) primes))

(defn prime-factors-of [num]
  "Returns the prime factors of a given number."
  (loop [a (lazy-primes-to (inc (/ num 2)))
         number num 
         factorlist '()]
    (if (seq a)
      (if (zero? (mod number (first a))) 
        (recur a (/ number (first a)) (conj factorlist (first a)))
        (recur (rest a) number factorlist))
      factorlist)))


(defn sum-divisors [num]
  "Returns sum of Proper Divisors (not including number itself)"
  (let [prime-factors (partition-by identity (prime-factors-of num))]
    (let [sum (reduce * (map #(if (> (count %) 1)
                                (-> (first %) (expt (inc (count %))) (dec) (/ (dec (first %))))
                                (inc (first %)))
                             prime-factors))]
      (if (= sum 1) 1 (- sum num)))))