diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 1a6eedd738c..f9414865ade 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -2347,6 +2347,9 @@ dict_update_common(PyObject *self, PyObject *args, PyObject *kwds,
     return result;
 }
 
+/* Note: dict.update() uses the METH_VARARGS|METH_KEYWORDS calling convention.
+   Using METH_FASTCALL would make dict.update(**dict2) calls slower, see the
+   issue #29312. */
 static PyObject *
 dict_update(PyObject *self, PyObject *args, PyObject *kwds)
 {