diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 5ebcfef6143..865dc1aa34e 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -3341,6 +3341,114 @@ test_critical_sections(PyObject *module, PyObject *Py_UNUSED(args))
     Py_RETURN_NONE;
 }
 
+static PyObject *
+bench_bytearray(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    Py_ssize_t loops;
+    if (!PyArg_ParseTuple(args, "n", &loops)) {
+        return NULL;
+    }
+
+    PyObject *str_var = PyBytes_FromString("var");
+    PyObject *str_equal = PyBytes_FromString("=");
+    PyObject *str_string_value = PyBytes_FromString("string value");
+    if (str_var == NULL || str_equal == NULL || str_string_value == NULL) {
+        Py_XDECREF(str_var);
+        Py_XDECREF(str_equal);
+        Py_XDECREF(str_string_value);
+        return NULL;
+    }
+
+    PyTime_t t1;
+    (void)PyTime_PerfCounterRaw(&t1);
+
+    for (Py_ssize_t i=0; i < loops; i++) {
+        PyObject *bytearray = PyByteArray_FromStringAndSize("", 0);
+        if (bytearray == NULL) {
+            return NULL;
+        }
+
+        PyObject *res;
+        res = PySequence_InPlaceConcat(bytearray, str_var);
+        if (res == NULL) {
+            return NULL;
+        }
+        Py_DECREF(res);
+
+        res = PySequence_InPlaceConcat(bytearray, str_equal);
+        if (res == NULL) {
+            return NULL;
+        }
+        Py_DECREF(res);
+
+        res = PySequence_InPlaceConcat(bytearray, str_string_value);
+        if (res == NULL) {
+            return NULL;
+        }
+        Py_DECREF(res);
+
+        char *str = PyByteArray_AsString(bytearray);
+        Py_ssize_t size = PyByteArray_Size(bytearray);
+        PyObject *bytes = PyBytes_FromStringAndSize(str, size);
+        Py_DECREF(bytearray);
+        assert(bytes != NULL);
+        Py_DECREF(bytes);
+    }
+
+    PyTime_t t2;
+    (void)PyTime_PerfCounterRaw(&t2);
+
+    Py_DECREF(str_var);
+    Py_DECREF(str_equal);
+    Py_DECREF(str_string_value);
+
+    return PyFloat_FromDouble(PyTime_AsSecondsDouble(t2 - t1));
+}
+
+
+static PyObject *
+bench_writer(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    Py_ssize_t loops;
+    if (!PyArg_ParseTuple(args, "n", &loops)) {
+        return NULL;
+    }
+
+    PyTime_t t1;
+    (void)PyTime_PerfCounterRaw(&t1);
+
+    for (Py_ssize_t i=0; i < loops; i++) {
+        char *str;
+        // 5 = len("var=x")
+        PyBytesWriter *writer = PyBytesWriter_Create(5, &str);
+        if (writer == NULL) {
+            return NULL;
+        }
+
+        memcpy(str, "var", 3);
+        str += 3;
+        *str++ = '=';
+
+        if (PyBytesWriter_Prepare(writer, &str, 12) < 0) {
+            PyBytesWriter_Discard(writer);
+            return NULL;
+        }
+
+        memcpy(str, "string value", 12);
+        str += 12;
+
+        PyObject *bytes = PyBytesWriter_Finish(writer, str);
+        assert(bytes != NULL);
+        Py_DECREF(bytes);
+    }
+
+    PyTime_t t2;
+    (void)PyTime_PerfCounterRaw(&t2);
+
+    return PyFloat_FromDouble(PyTime_AsSecondsDouble(t2 - t1));
+}
+
+
 static PyMethodDef TestMethods[] = {
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             test_config,                     METH_NOARGS},
@@ -3483,6 +3591,8 @@ static PyMethodDef TestMethods[] = {
     {"test_weakref_capi", test_weakref_capi, METH_NOARGS},
     {"function_set_warning", function_set_warning, METH_NOARGS},
     {"test_critical_sections", test_critical_sections, METH_NOARGS},
+    {"bench_bytearray", bench_bytearray, METH_VARARGS},
+    {"bench_writer", bench_writer, METH_VARARGS},
     {NULL, NULL} /* sentinel */
 };