diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -887,9 +887,10 @@
         else:
             seq_type_name = "sequence"
 
+        max_seq_len = 0
         differing = None
         try:
-            len1 = len(seq1)
+            len1 = max_seq_len = len(seq1)
         except (TypeError, NotImplementedError):
             differing = 'First %s has no length.    Non-sequence?' % (
                     seq_type_name)
@@ -900,6 +901,10 @@
             except (TypeError, NotImplementedError):
                 differing = 'Second %s has no length.    Non-sequence?' % (
                         seq_type_name)
+            else:
+                if len2 > max_seq_len:
+                    max_seq_len = len2
+
 
         if differing is None:
             if seq1 == seq2:
@@ -953,11 +958,14 @@
                     differing += ('Unable to index element %d '
                                   'of second %s\n' % (len1, seq_type_name))
         standardMsg = differing
-        diffMsg = '\n' + '\n'.join(
-            difflib.ndiff(pprint.pformat(seq1).splitlines(),
-                          pprint.pformat(seq2).splitlines()))
+        # calculating the diff might take a long time and produce unreadable
+        # output if there are too many elements
+        if max_seq_len < 30:
+            diffMsg = '\n' + '\n'.join(
+                difflib.ndiff(pprint.pformat(seq1).splitlines(),
+                              pprint.pformat(seq2).splitlines()))
+            standardMsg = self._truncateMessage(standardMsg, diffMsg)
 
-        standardMsg = self._truncateMessage(standardMsg, diffMsg)
         msg = self._formatMessage(msg, standardMsg)
         self.fail(msg)