PERF: Cache MultiIndex.levels

Closes pandas-dev#31648

Author: TomAugspurger

pandas/core/indexes/multi.py

@@ -677,8 +677,11 @@ def __len__(self) -> int:
     # --------------------------------------------------------------------
     # Levels Methods
 
-    @property
+    @cache_readonly
     def levels(self):
+        # Use cache_readonly to ensure that self.get_locs doesn't repeatedly
+        # create new IndexEngine
+        # https://github.com/pandas-dev/pandas/issues/31648
         result = [
             x._shallow_copy(name=name) for x, name in zip(self._levels, self._names)
         ]