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 11:22:37 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);
@@ -1679,10 +1679,25 @@
 static PyObject *
 set_isub(PySetObject *so, PyObject *other)
 {
+    int intersected = 0;
+
     if (!PyAnySet_Check(other))
         Py_RETURN_NOTIMPLEMENTED;
+
+    // if small -= big , optimize.  #issue8425
+    if (PySet_GET_SIZE(other) > 8*PySet_GET_SIZE(so)) {
+       other = set_intersection(so, other);
+       if (other == NULL)
+           return NULL;
+       else
+           intersected = 1;
+    }
+
     if (set_difference_update_internal(so, other) == -1)
         return NULL;
+
+    if (intersected)
+        Py_DECREF(other);
     Py_INCREF(so);
     return (PyObject *)so;
 }