diff -r 399d49d4acae Lib/xml/dom/minidom.py
--- a/Lib/xml/dom/minidom.py	Wed Jun 15 17:12:38 2016 -0500
+++ b/Lib/xml/dom/minidom.py	Thu Jun 16 16:45:44 2016 +0200
@@ -77,6 +77,13 @@
         if self.childNodes:
             return self.childNodes[-1]
 
+    def _index(self, refChild):
+        if self._index_cache >= 0 and self._index_cache < len(self.childNodes) \
+           and self.childNodes[self._index_cache] == refChild:
+            return self._index_cache
+        self._index_cache = self.childNodes.index(refChild)
+        return self._index_cache
+            
     def insertBefore(self, newChild, refChild):
         if newChild.nodeType == self.DOCUMENT_FRAGMENT_NODE:
             for c in tuple(newChild.childNodes):
@@ -92,12 +99,13 @@
             self.appendChild(newChild)
         else:
             try:
-                index = self.childNodes.index(refChild)
+                index = self._index(refChild)
             except ValueError:
                 raise xml.dom.NotFoundErr()
             if newChild.nodeType in _nodeTypes_with_children:
                 _clear_id_cache(self)
             self.childNodes.insert(index, newChild)
+            self._index_cache += 1
             newChild.nextSibling = refChild
             refChild.previousSibling = newChild
             if index:
@@ -139,7 +147,7 @@
         if newChild.parentNode is not None:
             newChild.parentNode.removeChild(newChild)
         try:
-            index = self.childNodes.index(oldChild)
+            index = self._index(oldChild)
         except ValueError:
             raise xml.dom.NotFoundErr()
         self.childNodes[index] = newChild
@@ -330,6 +338,7 @@
 
     def __init__(self):
         self.childNodes = NodeList()
+        self._index_cache = -1
 
 
 class Attr(Node):
@@ -647,6 +656,7 @@
         self.prefix = prefix
         self.namespaceURI = namespaceURI
         self.childNodes = NodeList()
+        self._index_cache = -1
 
         self._attrs = {}   # attributes are double-indexed:
         self._attrsNS = {} #    tagName -> Attribute
@@ -1306,6 +1316,7 @@
         self.nodeName = name
         self.notationName = notation
         self.childNodes = NodeList()
+        self._index_cache = -1
         self._identified_mixin_init(publicId, systemId)
 
     def _get_actualEncoding(self):
@@ -1499,6 +1510,7 @@
 
     def __init__(self):
         self.childNodes = NodeList()
+        self._index_cache = -1
         # mapping of (namespaceURI, localName) -> ElementInfo
         #        and tagName -> ElementInfo
         self._elem_info = {}