better merge

better_merge.clj

(require
  '[arcadia.internal.benchmarking :as b]
  '[magic.faster :as f])

(defn better-merge [m1 m2]
  (persistent!
    (reduce-kv assoc! (transient {}) m2)))

(let [m {}
      m2 (zipmap (range 1e4) (range))]
  (f/faster
    (b/n-timing 10
      (merge m m2))))

;; ~59

(let [m {}
      m2 (zipmap (range 1e4) (range))]
  (f/faster
    (b/n-timing 10
      (better-merge m m2))))

;; ~25