PERF: DataFrame.T (#44724)

Author: jbrockmendel

pandas/core/internals/construction.py

@@ -295,7 +295,7 @@ def ndarray_to_mgr(
     copy_on_sanitize = False if typ == "array" else copy
 
     vdtype = getattr(values, "dtype", None)
-    if is_1d_only_ea_dtype(vdtype) or isinstance(dtype, ExtensionDtype):
+    if is_1d_only_ea_dtype(vdtype) or is_1d_only_ea_dtype(dtype):
         # GH#19157
 
         if isinstance(values, (np.ndarray, ExtensionArray)) and values.ndim > 1:

pandas/tests/frame/test_constructors.py

@@ -70,13 +70,16 @@
 
 
 class TestDataFrameConstructors:
-    def test_constructor_from_2d_datetimearray(self):
+    def test_constructor_from_2d_datetimearray(self, using_array_manager):
         dti = date_range("2016-01-01", periods=6, tz="US/Pacific")
         dta = dti._data.reshape(3, 2)
 
         df = DataFrame(dta)
         expected = DataFrame({0: dta[:, 0], 1: dta[:, 1]})
         tm.assert_frame_equal(df, expected)
+        if not using_array_manager:
+            # GH#44724 big performance hit if we de-consolidate
+            assert len(df._mgr.blocks) == 1
 
     def test_constructor_dict_with_tzaware_scalar(self):
         # GH#42505