diff -r 178f9042af81 Objects/setobject.c
--- a/Objects/setobject.c	Sun Sep 23 11:11:07 2012 +0200
+++ b/Objects/setobject.c	Mon Sep 24 22:02:13 2012 +0200
@@ -834,7 +834,7 @@
     /* copy the itertor state */
     tmp = *si;
     Py_XINCREF(tmp.si_set);
-    
+
     /* iterate the temporary into a list */
     for(;;) {
         PyObject *element = setiter_iternext(&tmp);
@@ -1529,10 +1529,24 @@
     if (PyAnySet_Check(other)) {
         setentry *entry;
         Py_ssize_t pos = 0;
+        int intersected = 0;
+
+        // if small -= big , optimize.  #issue8425
+        if (PySet_GET_SIZE(other) > 8*PySet_GET_SIZE(so)) {
+            other = set_intersection(so, other);
+            if (other == NULL)
+                return -1;
+            else
+                intersected = 1;
+        }
 
         while (set_next((PySetObject *)other, &pos, &entry))
             if (set_discard_entry(so, entry) == -1)
                 return -1;
+
+        if (intersected)
+            Py_DECREF(other);
+
     } else {
         PyObject *key, *it;
         it = PyObject_GetIter(other);