diff --git a/bench/src/datascript/bench.cljc b/bench/src/datascript/bench.cljc
index e806c592..1f6fadf2 100644
--- a/bench/src/datascript/bench.cljc
+++ b/bench/src/datascript/bench.cljc
@@ -130,7 +130,9 @@
         (contains? set i)))
     (perf/bench {:target tn :test "set-iterate" :size size}
       (doseq [x set]
-        (+ 1 x)))))
+        (+ 1 x)))
+    (perf/bench {:target tn :test "set-reduce" :size size}
+      (reduce + 0 set))))
 
 (defn ^:export bench-all []
   (bench-db_with)
diff --git a/src/datascript/btset.cljc b/src/datascript/btset.cljc
index f1da5a37..efcaf686 100644
--- a/src/datascript/btset.cljc
+++ b/src/datascript/btset.cljc
@@ -96,19 +96,14 @@
   (shim/aget arr (dec (shim/alength arr))))
 
 (defn cut-n-splice [arr cut-from cut-to splice-from splice-to xs]
-  ;; TODO arraycopy
-  (let [arr-l   (shim/alength arr)
-        xs-l    (shim/alength xs)
-        l1      (- splice-from cut-from)
-        l2      (- cut-to splice-to)
-        l1xs    (+ l1 xs-l)
+  (let [xs-l (shim/alength xs)
+        l1   (- splice-from cut-from)
+        l2   (- cut-to splice-to)
+        l1xs (+ l1 xs-l)
         new-arr (shim/make-array (+ l1 xs-l l2))]
-    (dotimes [i l1]
-      (shim/aset new-arr i (shim/aget arr (+ cut-from i))))
-    (dotimes [i xs-l]
-      (shim/aset new-arr (+ i l1) (shim/aget xs i)))
-    (dotimes [i l2]
-      (shim/aset new-arr (+ i l1xs) (shim/aget arr (+ splice-to i))))
+    (shim/acopy arr cut-from splice-from new-arr 0)
+    (shim/acopy xs 0 xs-l new-arr l1)
+    (shim/acopy arr splice-to cut-to new-arr l1xs)
     new-arr))
 
 (defn cut
@@ -130,7 +125,6 @@
   (cut-n-splice arr 0 (shim/alength arr) idx idx xs))
 
 (defn merge-n-split [a1 a2]
-  ;; TODO arraycopy
   (let [a1-l    (shim/alength a1)
         a2-l    (shim/alength a2)
         total-l (+ a1-l a2-l)
@@ -138,13 +132,15 @@
         r2-l    (- total-l r1-l)
         r1      (shim/make-array r1-l)
         r2      (shim/make-array r2-l)]
-    (dotimes [i total-l]
-      (let [set-a (if (< i r1-l) r1 r2)
-            set-i (if (< i r1-l) i (- i r1-l))
-            get-a (if (< i a1-l) a1 a2)
-            get-i (if (< i a1-l) i (- i a1-l))
-            set-v (shim/aget get-a get-i)]
-        (shim/aset set-a set-i set-v)))
+    (if (<= a1-l r1-l)
+      (do
+        (shim/acopy a1 0             a1-l          r1 0)
+        (shim/acopy a2 0             (- r1-l a1-l) r1 a1-l)
+        (shim/acopy a2 (- r1-l a1-l) a2-l          r2 0))
+      (do
+        (shim/acopy a1 0    r1-l r1 0)
+        (shim/acopy a1 r1-l a1-l r2 0)
+        (shim/acopy a2 0    a2-l r2 (- a1-l r1-l))))
     (shim/array r1 r2)))
 
 (defn ^boolean eq-arr [cmp a1 a1-from a1-to
diff --git a/src/datascript/shim.cljc b/src/datascript/shim.cljc
index cfa32641..b1a32d37 100644
--- a/src/datascript/shim.cljc
+++ b/src/datascript/shim.cljc
@@ -44,10 +44,22 @@
         (list* 'js* (str "[" (str/join "," (repeat (count args) "~{}")) "]") args)
         (vary-meta assoc :tag 'array))
       (let [len (count args)]
-       `(let [arr# (clojure.core/make-array java.lang.Object ~len)]
-          (doto ^{:tag "[[Ljava.lang.Object;"} arr#
-          ~@(map #(list 'aset % (nth args %)) (range len))))))))
+        (if (zero? len)
+          'clojure.lang.RT/EMPTY_ARRAY
+         `(let [arr# (clojure.core/make-array java.lang.Object ~len)]
+            (doto ^{:tag "[[Ljava.lang.Object;"} arr#
+            ~@(map #(list 'aset % (nth args %)) (range len)))))))))
 
+#?(:clj
+  (defmacro acopy [from from-start from-end to to-start]
+    (if-cljs &env
+     `(let [l# (- ~from-end ~from-start)]
+        (dotimes [i# l#]
+          (aset ~to (+ i# ~to-start) (aget ~from (+ i# ~from-start)))))
+     `(let [l# (- ~from-end ~from-start)]
+        (when (pos? l#)
+          (System/arraycopy ~from ~from-start ~to ~to-start l#))))))
+      
 
 (defn aconcat [a b]
   #?(:cljs (.concat a b)