PERF: speed up MultiIndex.is_monotonic by 50x

Author: qwhelan

pandas/core/indexes/multi.py

@@ -1359,6 +1359,9 @@ def is_monotonic_increasing(self):
         increasing) values.
         """
 
+        if all([x.is_monotonic for x in self.levels]):
+            return libalgos.is_lexsorted([x.astype("int64") for x in self.codes])
+
         # reversed() because lexsort() wants the most significant key last.
         values = [
             self._get_level_values(i).values for i in reversed(range(len(self.levels)))