Improve performance when selecting rows and columns (pandas-dev#53014)

* Improve performance when selecting rows and columns

* Update doc/source/whatsnew/v2.1.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Update indexing.py

* Update v2.1.0.rst

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: 2 people

doc/source/whatsnew/v2.1.0.rst

@@ -285,6 +285,7 @@ Performance improvements
 - Performance improvement accessing :attr:`arrays.IntegerArrays.dtype` & :attr:`arrays.FloatingArray.dtype` (:issue:`52998`)
 - Performance improvement in :class:`Series` reductions (:issue:`52341`)
 - Performance improvement in :func:`concat` when ``axis=1`` and objects have different indexes (:issue:`52541`)
+- Performance improvement in :meth:`DataFrame.loc` when selecting rows and columns (:issue:`53014`)
 - Performance improvement in :meth:`Series.corr` and :meth:`Series.cov` for extension dtypes (:issue:`52502`)
 - Performance improvement in :meth:`Series.to_numpy` when dtype is a numpy float dtype and ``na_value`` is ``np.nan`` (:issue:`52430`)
 - Performance improvement in :meth:`~arrays.ArrowExtensionArray.to_numpy` (:issue:`52525`)

pandas/core/indexing.py

@@ -933,7 +933,9 @@ def _getitem_tuple_same_dim(self, tup: tuple):
         This is only called after a failed call to _getitem_lowerdim.
         """
         retval = self.obj
-        for i, key in enumerate(tup):
+        # Selecting columns before rows is signficiantly faster
+        for i, key in enumerate(reversed(tup)):
+            i = self.ndim - i - 1
             if com.is_null_slice(key):
                 continue