diff -r b7c0137cccbe Objects/setobject.c
--- a/Objects/setobject.c	Thu Mar 26 23:50:57 2015 +0100
+++ b/Objects/setobject.c	Fri Mar 27 09:29:40 2015 +0200
@@ -551,13 +551,14 @@ set_merge(PySetObject *so, PyObject *oth
     Py_hash_t hash;
     Py_ssize_t i;
     setentry *entry;
+    int clear, dump;
 
     assert (PyAnySet_Check(so));
     assert (PyAnySet_Check(otherset));
 
     other = (PySetObject*)otherset;
     if (other == so || other->used == 0)
-        /* a.update(a) or a.update({}); nothing to do */
+        /* a.update(a) or a.update(set()); nothing to do */
         return 0;
     /* Do one big resize at the start, rather than
      * incrementally resizing as we insert new keys.  Expect
@@ -567,6 +568,10 @@ set_merge(PySetObject *so, PyObject *oth
        if (set_table_resize(so, (so->used + other->used)*2) != 0)
            return -1;
     }
+    /* If the table is clear, we can use faster set_insert_clean(). */
+    clear = (so->fill == 0);
+    /* If no dummies, we can just copy entries at the same indices. */
+    dump = (other->fill == other->used && so->mask == other->mask);
     for (i = 0; i <= other->mask; i++) {
         entry = &other->table[i];
         key = entry->key;
@@ -574,9 +579,23 @@ set_merge(PySetObject *so, PyObject *oth
         if (key != NULL &&
             key != dummy) {
             Py_INCREF(key);
-            if (set_insert_key(so, key, hash) == -1) {
-                Py_DECREF(key);
-                return -1;
+            if (clear) {
+                if (dump) {
+                    entry = &so->table[i];
+                    assert(entry->key == NULL);
+                    entry->key = key;
+                    entry->hash = hash;
+                    so->used++;
+                    so->fill++;
+                }
+                else
+                    set_insert_clean(so, key, hash);
+            }
+            else {
+                if (set_insert_key(so, key, hash) == -1) {
+                    Py_DECREF(key);
+                    return -1;
+                }
             }
         }
     }