diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index bcdb60d..4a1362d 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/Doc/whatsnew/3.8.rst
@@ -1090,8 +1090,9 @@
   <http://blog.unicode.org/2019/05/unicode-12-1-en.html>`_ release.
 
 * New function :func:`~unicodedata.is_normalized` can be used to verify a string
-  is in a specific normal form. (Contributed by Max Belanger and David Euresti in
-  :issue:`32285`).
+  is in a specific normal form, often much faster than by actually normalizing
+  the string.  (Contributed by Max Belanger, David Euresti, and Greg Price in
+  :issue:`32285` and :issue:`37966`).
 
 
 unittest
diff --git a/Lib/test/test_unicodedata.py b/Lib/test/test_unicodedata.py
index a52b6de..07d7176 100644
--- a/Lib/test/test_unicodedata.py
+++ b/Lib/test/test_unicodedata.py
@@ -220,6 +220,8 @@
         self.assertEqual(self.db.normalize('NFC', u11a7_str_a), u11a7_str_b)
         self.assertEqual(self.db.normalize('NFC', u11c3_str_a), u11c3_str_b)
 
+    # For tests of unicodedata.is_normalized / self.db.is_normalized ,
+    # see test_normalization.py .
 
     def test_east_asian_width(self):
         eaw = self.db.east_asian_width
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-08-27-21-21-36.bpo-37966.5OBLez.rst b/Misc/NEWS.d/next/Core and Builtins/2019-08-27-21-21-36.bpo-37966.5OBLez.rst
new file mode 100644
index 0000000..6b9d69c
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-08-27-21-21-36.bpo-37966.5OBLez.rst
@@ -0,0 +1,3 @@
+The implementation of :func:`~unicodedata.is_normalized` has been greatly
+sped up on strings that aren't normalized, by implementing the full
+normalization-quick-check algorithm from the Unicode standard.
diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
index ae0d4e4..5e8ba60 100644
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -19,6 +19,8 @@
 #include "ucnhash.h"
 #include "structmember.h"
 
+#include <stdbool.h>
+
 _Py_IDENTIFIER(NFC);
 _Py_IDENTIFIER(NFD);
 _Py_IDENTIFIER(NFKC);
@@ -775,25 +777,40 @@
     return result;
 }
 
-typedef enum {YES, NO, MAYBE} NormalMode;
+// This needs to match the logic in makeunicodedata.py
+// which constructs the quickcheck data.
+typedef enum {YES = 0, MAYBE = 1, NO = 2} QuickcheckResult;
 
-/* Return YES if the input is certainly normalized, NO or MAYBE if it might not be. */
-static NormalMode
-is_normalized(PyObject *self, PyObject *input, int nfc, int k)
+/* Run the Unicode normalization "quickcheck" algorithm.
+ *
+ * Return YES or NO if quickcheck determines the input is certainly
+ * normalized or certainly not, and MAYBE if quickcheck is unable to
+ * tell.
+ *
+ * If `yes_only` is true, then return MAYBE as soon as we determine
+ * the answer is not YES.
+ *
+ * For background and details on the algorithm, see UAX #15:
+ *   https://www.unicode.org/reports/tr15/#Detecting_Normalization_Forms
+ */
+static QuickcheckResult
+is_normalized_quickcheck(PyObject *self, PyObject *input,
+                         int nfc, int k, bool yes_only)
 {
-    Py_ssize_t i, len;
-    int kind;
-    void *data;
-    unsigned char prev_combining = 0, quickcheck_mask;
-
     /* An older version of the database is requested, quickchecks must be
        disabled. */
     if (self && UCD_Check(self))
         return NO;
 
-    /* The two quickcheck bits at this shift mean 0=Yes, 1=Maybe, 2=No,
-       as described in http://unicode.org/reports/tr15/#Annex8. */
-    quickcheck_mask = 3 << ((nfc ? 4 : 0) + (k ? 2 : 0));
+    Py_ssize_t i, len;
+    int kind;
+    void *data;
+    unsigned char prev_combining = 0;
+
+    /* The two quickcheck bits at this shift have type QuickcheckResult. */
+    int quickcheck_shift = (nfc ? 4 : 0) + (k ? 2 : 0);
+
+    QuickcheckResult result = YES; /* certainly normalized, unless we find something */
 
     i = 0;
     kind = PyUnicode_KIND(input);
@@ -802,16 +819,26 @@
     while (i < len) {
         Py_UCS4 ch = PyUnicode_READ(kind, data, i++);
         const _PyUnicode_DatabaseRecord *record = _getrecord_ex(ch);
-        unsigned char combining = record->combining;
-        unsigned char quickcheck = record->normalization_quick_check;
 
-        if (quickcheck & quickcheck_mask)
-            return MAYBE; /* this string might need normalization */
+        unsigned char combining = record->combining;
         if (combining && prev_combining > combining)
             return NO; /* non-canonical sort order, not normalized */
         prev_combining = combining;
+
+        unsigned char quickcheck_whole = record->normalization_quick_check;
+        if (yes_only) {
+            if (quickcheck_whole & (3 << quickcheck_shift))
+                return MAYBE;
+        } else {
+            switch ((quickcheck_whole >> quickcheck_shift) & 3) {
+            case NO:
+              return NO;
+            case MAYBE:
+              result = MAYBE; /* this string might need normalization */
+            }
+        }
     }
-    return YES; /* certainly normalized */
+    return result;
 }
 
 /*[clinic input]
@@ -844,7 +871,7 @@
     PyObject *result;
     int nfc = 0;
     int k = 0;
-    NormalMode m;
+    QuickcheckResult m;
 
     PyObject *cmp;
     int match = 0;
@@ -867,7 +894,7 @@
         return NULL;
     }
 
-    m = is_normalized(self, input, nfc, k);
+    m = is_normalized_quickcheck(self, input, nfc, k, false);
 
     if (m == MAYBE) {
         cmp = (nfc ? nfc_nfkc : nfd_nfkd)(self, input, k);
@@ -913,28 +940,28 @@
     }
 
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFC)) {
-        if (is_normalized(self, input, 1, 0) == YES) {
+        if (is_normalized_quickcheck(self, input, 1, 0, true) == YES) {
             Py_INCREF(input);
             return input;
         }
         return nfc_nfkc(self, input, 0);
     }
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFKC)) {
-        if (is_normalized(self, input, 1, 1) == YES) {
+        if (is_normalized_quickcheck(self, input, 1, 1, true) == YES) {
             Py_INCREF(input);
             return input;
         }
         return nfc_nfkc(self, input, 1);
     }
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFD)) {
-        if (is_normalized(self, input, 0, 0) == YES) {
+        if (is_normalized_quickcheck(self, input, 0, 0, true) == YES) {
             Py_INCREF(input);
             return input;
         }
         return nfd_nfkd(self, input, 0);
     }
     if (_PyUnicode_EqualToASCIIId(form, &PyId_NFKD)) {
-        if (is_normalized(self, input, 0, 1) == YES) {
+        if (is_normalized_quickcheck(self, input, 0, 1, true) == YES) {
             Py_INCREF(input);
             return input;
         }