PERF: restore DatetimeIndex.__iter__ performance by using non-EA implementation

Author: qwhelan

pandas/core/indexes/datetimes.py

@@ -243,6 +243,8 @@ def _join_i8_wrapper(joinf, **kwargs):
     _is_numeric_dtype = False
     _infer_as_myclass = True
 
+    # Use faster implementation given we know we have DatetimeArrays
+    __iter__ = DatetimeArray.__iter__
     # some things like freq inference make use of these attributes.
     _bool_ops = DatetimeArray._bool_ops
     _object_ops = DatetimeArray._object_ops