diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -968,10 +968,14 @@
 
    Some common encodings can bypass the codecs lookup machinery to
    improve performance.  These optimization opportunities are only
-   recognized by CPython for a limited set of aliases: utf-8, utf8,
-   latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii, utf-16,
-   and utf-32.  Using alternative spellings for these encodings may
-   result in slower execution.
+   recognized by CPython for a limited set of (case insensitive) aliases:
+   utf-8, utf8, latin-1, latin1, iso-8859-1, mbcs (Windows only), ascii,
+   us-ascii, utf-16, and utf-32, and the same using underscores instead of
+   dashes. Using alternative aliases for these encodings may result in slower
+   execution.
+
+   .. versionadded:: 3.6
+      Optimization opportunity recognized for us-ascii.
 
 Many of the character sets support the same languages. They vary in individual
 characters (e.g. whether the EURO SIGN is supported or not), and in the