PERF: add shortcut to Timestamp constructor

doc/source/whatsnew/v1.0.0.rst

@@ -775,6 +775,7 @@ Performance improvements
   The improvement is not present if checking if the :class:`Categorical` is less than or less than or equal than the scalar (:issue:`29820`)
 - Performance improvement in :meth:`Index.equals` and  :meth:`MultiIndex.equals` (:issue:`29134`)
 - Performance improvement in :func:`~pandas.api.types.infer_dtype` when ``skipna`` is ``True`` (:issue:`28814`)
+- Performance improvement in :class:`Timestamp` constructor (:issue:`30543`)
 
 .. _whatsnew_1000.bug_fixes:
 

pandas/_libs/tslibs/timestamps.pyx

@@ -389,7 +389,10 @@ class Timestamp(_Timestamp):
             # User passed tzinfo instead of tz; avoid silently ignoring
             tz, tzinfo = tzinfo, None
 
-        if isinstance(ts_input, str):
+        if isinstance(ts_input, Timestamp) and tz is None:
+            # GH 30543 if pd.Timestamp already passed, return it
+            return ts_input
+        elif isinstance(ts_input, str):
             # User passed a date string to parse.
             # Check that the user didn't also pass a date attribute kwarg.
             if any(arg is not None for arg in _date_attributes):

pandas/tests/indexes/datetimes/test_tools.py

@@ -2313,3 +2313,10 @@ def test_nullable_integer_to_datetime():
     tm.assert_series_equal(res, expected)
     # Check that ser isn't mutated
     tm.assert_series_equal(ser, ser_copy)
+
+
+def test_timestamp_constructor_identity():
+    # Test for #30543
+    expected = pd.Timestamp("2017-01-01T12")
+    result = pd.Timestamp(expected)
+    assert result is expected