diff --git a/lib/internal/mime.js b/lib/internal/mime.js
index efe6a2212b0483..de691a745031e0 100644
--- a/lib/internal/mime.js
+++ b/lib/internal/mime.js
@@ -23,6 +23,8 @@ const END_BEGINNING_WHITESPACE = /[^\r\n\t ]|$/;
 const START_ENDING_WHITESPACE = /[\r\n\t ]*$/;
 
 function toASCIILower(str) {
+  // eslint-disable-next-line no-control-regex
+  if (!/[^\x00-\x7f]/.test(str)) return StringPrototypeToLowerCase(str);
   let result = '';
   for (let i = 0; i < str.length; i++) {
     const char = str[i];