diff --git a/lib/utils/strings.js b/lib/utils/strings.js
index 868c1694..c0227e0e 100644
--- a/lib/utils/strings.js
+++ b/lib/utils/strings.js
@@ -8,6 +8,9 @@ exports.asciiWhitespaceRe = asciiWhitespaceRe;
 
 // https://infra.spec.whatwg.org/#ascii-lowercase
 exports.asciiLowercase = (s) => {
+  if (!/[^\x00-\x7f]/.test(s)) {
+    return s.toLowerCase();
+  }
   const len = s.length;
   const out = new Array(len);
   for (let i = 0; i < len; i++) {
@@ -20,6 +23,9 @@ exports.asciiLowercase = (s) => {
 
 // https://infra.spec.whatwg.org/#ascii-uppercase
 exports.asciiUppercase = (s) => {
+  if (!/[^\x00-\x7f]/.test(s)) {
+    return s.toUpperCase();
+  }
   const len = s.length;
   const out = new Array(len);
   for (let i = 0; i < len; i++) {