diff -r b15c5a66213f Objects/abstract.c
--- a/Objects/abstract.c	Thu Oct 02 12:39:02 2014 +0200
+++ b/Objects/abstract.c	Thu Oct 02 13:28:41 2014 +0200
@@ -2557,6 +2557,11 @@ PyObject_IsInstance(PyObject *inst, PyOb
         return r;
     }
 
+    if (Py_TYPE(cls) == &PyType_Type) {
+        /* We know what type's __instancecheck__ does. */
+	return recursive_isinstance(inst, cls);
+    }
+
     checker = _PyObject_LookupSpecial(cls, &PyId___instancecheck__);
     if (checker != NULL) {
         PyObject *res;
@@ -2576,6 +2581,7 @@ PyObject_IsInstance(PyObject *inst, PyOb
     }
     else if (PyErr_Occurred())
         return -1;
+    /* Probably never reached anymore. */
     return recursive_isinstance(inst, cls);
 }
 
@@ -2622,6 +2628,11 @@ PyObject_IsSubclass(PyObject *derived, P
         return r;
     }
 
+    if (Py_TYPE(cls) == &PyType_Type) {
+        /* We know what type's __subclasscheck__ does. */
+	return recursive_issubclass(derived, cls);
+    }
+
     checker = _PyObject_LookupSpecial(cls, &PyId___subclasscheck__);
     if (checker != NULL) {
         PyObject *res;
@@ -2641,6 +2652,7 @@ PyObject_IsSubclass(PyObject *derived, P
     }
     else if (PyErr_Occurred())
         return -1;
+    /* Probably never reached anymore. */
     return recursive_issubclass(derived, cls);
 }