diff -r a8563ef0eb8a Objects/dictobject.c
--- a/Objects/dictobject.c	Tue Jan 17 10:07:25 2017 +0200
+++ b/Objects/dictobject.c	Tue Jan 17 17:10:54 2017 +0900
@@ -3296,11 +3296,71 @@ PyObject *
 
 /* For backward compatibility with old dictionary interface */
 
+/* PyDict_GetItemString specialized for key is ascii string.
+ *
+ * Implementation is same to lookdict_unicode, except comparing only ascii
+ * C string.
+ */
+static PyObject *
+dict_getitemascii(PyDictObject *mp, const char *key, Py_ssize_t len)
+{
+    assert(mp->ma_keys->dk_lookup != lookdict);
+
+    Py_hash_t hash = _Py_HashBytes(key, len);
+    size_t mask = DK_MASK(mp->ma_keys);
+    size_t perturb = hash;
+    size_t i = hash & mask;
+    PyDictKeyEntry *ep0 = DK_ENTRIES(mp->ma_keys);
+
+    for (;;) {
+        Py_ssize_t ix = dk_get_index(mp->ma_keys, i);
+        if (ix == DKIX_EMPTY) {
+            return NULL;
+        }
+        if (ix >= 0) {
+            PyDictKeyEntry *ep = &ep0[ix];
+            PyObject *ekey = ep->me_key;
+            assert(PyUnicode_IS_READY(ekey));
+
+            if (ep->me_hash == hash && PyUnicode_IS_ASCII(ekey) &&
+                    PyUnicode_GET_LENGTH(ekey) == len &&
+                    memcmp(PyUnicode_DATA(ekey), key, len) == 0) {
+                if (_PyDict_HasSplitTable(mp)) {
+                    return mp->ma_values[ix];
+                }
+                else {
+                    return ep->me_value;
+                }
+            }
+        }
+
+        perturb >>= PERTURB_SHIFT;
+        i = mask & ((i << 2) + i + perturb + 1);
+    }
+    assert(0);
+    return NULL;
+}
+
 PyObject *
 PyDict_GetItemString(PyObject *v, const char *key)
 {
+    if (!PyDict_Check(v))
+        return NULL;
+
+    Py_ssize_t len;
+    int ascii = 1;
+    for (len = 0; key[len]; len++) {
+        if (key[len] < 0) {
+            ascii = 0;
+        }
+    }
+
+    if (ascii && ((PyDictObject*)v)->ma_keys->dk_lookup != lookdict) {
+        return dict_getitemascii((PyDictObject*)v, key, len);
+    }
+
     PyObject *kv, *rv;
-    kv = PyUnicode_FromString(key);
+    kv = PyUnicode_FromStringAndSize(key, len);
     if (kv == NULL) {
         PyErr_Clear();
         return NULL;