diff --git a/doc/source/whatsnew/v0.23.2.txt b/doc/source/whatsnew/v0.23.2.txt
index b8d865195cddd..f7e170cca039e 100644
--- a/doc/source/whatsnew/v0.23.2.txt
+++ b/doc/source/whatsnew/v0.23.2.txt
@@ -27,6 +27,7 @@ Performance Improvements
 - Improved performance of membership checks in :class:`CategoricalIndex`
   (i.e. ``x in ci``-style checks are much faster). :meth:`CategoricalIndex.contains`
   is likewise much faster (:issue:`21369`)
+- Improved performance of :meth:`MultiIndex.is_unique` (:issue:`21522`)
 -
 
 Documentation Changes
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 75b6be96feb78..ab23a80acdaae 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -852,14 +852,6 @@ def _has_complex_internals(self):
         # to disable groupby tricks
         return True
 
-    @cache_readonly
-    def is_monotonic(self):
-        """
-        return if the index is monotonic increasing (only equal or
-        increasing) values.
-        """
-        return self.is_monotonic_increasing
-
     @cache_readonly
     def is_monotonic_increasing(self):
         """
@@ -887,10 +879,6 @@ def is_monotonic_decreasing(self):
         # monotonic decreasing if and only if reverse is monotonic increasing
         return self[::-1].is_monotonic_increasing
 
-    @cache_readonly
-    def is_unique(self):
-        return not self.duplicated().any()
-
     @cache_readonly
     def _have_mixed_levels(self):
         """ return a boolean list indicated if we have mixed levels """