Index: src/wp-admin/js/password-strength-meter.js
===================================================================
--- src/wp-admin/js/password-strength-meter.js	(revision 32058)
+++ src/wp-admin/js/password-strength-meter.js	(working copy)
@@ -18,7 +18,10 @@
 			if (password1 != password2 && password2 && password2.length > 0)
 				return 5;
 
-			var result = zxcvbn( password1, blacklist );
+			// Analyze the strength of the first 128 characters.  The 'zxcvbn' library takes a long time to analyse longer passwords.  This causes the browser UI to freeze.
+			// @todo Update zxcvbn once it analyzes long passwords quickly.
+			// @see https://github.com/dropbox/zxcvbn/issues/69
+			var result = zxcvbn( password1.substr(0, 128), blacklist );
 			return result.score;
 		},