PERF: CategoricalDtype.__eq__ (pandas-dev#36280)

Author: jbrockmendel

pandas/core/dtypes/dtypes.py

@@ -375,12 +375,30 @@ def __eq__(self, other: Any) -> bool:
             # but same order is not necessary.  There is no distinction between
             # ordered=False and ordered=None: CDT(., False) and CDT(., None)
             # will be equal if they have the same categories.
-            if (
-                self.categories.dtype == other.categories.dtype
-                and self.categories.equals(other.categories)
-            ):
+            left = self.categories
+            right = other.categories
+
+            # GH#36280 the ordering of checks here is for performance
+            if not left.dtype == right.dtype:
+                return False
+
+            if len(left) != len(right):
+                return False
+
+            if self.categories.equals(other.categories):
                 # Check and see if they happen to be identical categories
                 return True
+
+            if left.dtype != object:
+                # Faster than calculating hash
+                indexer = left.get_indexer(right)
+                # Because left and right have the same length and are unique,
+                #  `indexer` not having any -1s implies that there is a
+                #  bijection between `left` and `right`.
+                return (indexer != -1).all()
+
+            # With object-dtype we need a comparison that identifies
+            #  e.g. int(2) as distinct from float(2)
             return hash(self) == hash(other)
 
     def __repr__(self) -> str_type: