From a4b0d7322e6fc493f74e699b99b3e5d734f2d912 Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Fri, 14 Oct 2022 20:13:18 -0400
Subject: [PATCH 1/4] perf: string array from np.str_

---
 asv_bench/benchmarks/array.py | 18 ++++++++++++++++++
 pandas/_libs/lib.pyx          |  4 ++++
 2 files changed, 22 insertions(+)

diff --git a/asv_bench/benchmarks/array.py b/asv_bench/benchmarks/array.py
index df9d171a70397..a8d5777216242 100644
--- a/asv_bench/benchmarks/array.py
+++ b/asv_bench/benchmarks/array.py
@@ -44,6 +44,24 @@ def time_from_integer_array(self):
         pd.array(self.values_integer, dtype="Int64")
 
 
+class StringArray:
+    def setup(self):
+        N = 100_000
+        values = tm.rands_array(3, N)
+        self.values_obj = np.array(values, dtype="object")
+        self.values_str = np.array(values, dtype="U")
+        self.values_list = values.tolist()
+
+    def time_from_np_object_array(self):
+        pd.array(self.values_obj, dtype="string")
+
+    def time_from_np_str_array(self):
+        pd.array(self.values_str, dtype="string")
+
+    def time_from_list(self):
+        pd.array(self.values_list, dtype="string")
+
+
 class ArrowStringArray:
 
     params = [False, True]
diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
index d9a7195520fd7..2f1473e10a5a3 100644
--- a/pandas/_libs/lib.pyx
+++ b/pandas/_libs/lib.pyx
@@ -703,6 +703,10 @@ cpdef ndarray[object] ensure_string_array(
     if copy and result is arr:
         result = result.copy()
 
+    if util.is_array(arr) and issubclass(arr.dtype.type, np.str_):
+        # short-circuit, all elements are str
+        return result
+
     for i in range(n):
         val = arr[i]
 

From 172ce4497ff6b55c171867d67dabc2fc60c71861 Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Fri, 14 Oct 2022 20:43:50 -0400
Subject: [PATCH 2/4] add test

---
 pandas/tests/arrays/string_/test_string.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/pandas/tests/arrays/string_/test_string.py b/pandas/tests/arrays/string_/test_string.py
index a7b8162eb981a..2a8708403a811 100644
--- a/pandas/tests/arrays/string_/test_string.py
+++ b/pandas/tests/arrays/string_/test_string.py
@@ -611,3 +611,11 @@ def test_setitem_scalar_with_mask_validation(dtype):
         msg = "Scalar must be NA or str"
     with pytest.raises(ValueError, match=msg):
         ser[mask] = 1
+
+
+def test_from_numpy_str(dtype):
+    vals = ["a", "b", "c"]
+    arr = np.array(vals, dtype=np.str_)
+    result = pd.array(arr, dtype=dtype)
+    expected = pd.array(vals, dtype=dtype)
+    tm.assert_extension_array_equal(result, expected)

From 3617d97b6a978e9aed97431a7ff6319b95022169 Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Fri, 14 Oct 2022 20:51:32 -0400
Subject: [PATCH 3/4] whatsnew

---
 doc/source/whatsnew/v2.0.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v2.0.0.rst b/doc/source/whatsnew/v2.0.0.rst
index fea3d70d81554..c1530598616cb 100644
--- a/doc/source/whatsnew/v2.0.0.rst
+++ b/doc/source/whatsnew/v2.0.0.rst
@@ -154,6 +154,7 @@ Performance improvements
 - Performance improvement in :func:`merge` and :meth:`DataFrame.join` when joining on a sorted :class:`MultiIndex` (:issue:`48504`)
 - Performance improvement in :meth:`DataFrame.loc` and :meth:`Series.loc` for tuple-based indexing of a :class:`MultiIndex` (:issue:`48384`)
 - Performance improvement for :meth:`MultiIndex.unique` (:issue:`48335`)
+- Performance improvement for :class:`~arrays.StringArray` constructor passing a numpy array with type ``np.str_`` (:issue:`49109`)
 - Performance improvement in :meth:`DataFrame.join` when joining on a subset of a :class:`MultiIndex` (:issue:`48611`)
 - Performance improvement for :meth:`MultiIndex.intersection` (:issue:`48604`)
 - Performance improvement in ``var`` for nullable dtypes (:issue:`48379`).

From 514153640ab51017f05a4b7473b224bbec7ac7fd Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Mon, 17 Oct 2022 19:04:26 -0400
Subject: [PATCH 4/4] cleanup

---
 pandas/_libs/lib.pyx | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
index 2f1473e10a5a3..de226fcd19084 100644
--- a/pandas/_libs/lib.pyx
+++ b/pandas/_libs/lib.pyx
@@ -703,7 +703,7 @@ cpdef ndarray[object] ensure_string_array(
     if copy and result is arr:
         result = result.copy()
 
-    if util.is_array(arr) and issubclass(arr.dtype.type, np.str_):
+    if issubclass(arr.dtype.type, np.str_):
         # short-circuit, all elements are str
         return result