diff --git a/lib/internal/mime.js b/lib/internal/mime.js
index de691a745031e0..b1df776e771961 100644
--- a/lib/internal/mime.js
+++ b/lib/internal/mime.js
@@ -7,6 +7,7 @@ const {
   SafeMap,
   SafeStringPrototypeSearch,
   StringPrototypeCharAt,
+  StringPrototypeCharCodeAt,
   StringPrototypeIndexOf,
   StringPrototypeSlice,
   StringPrototypeToLowerCase,
@@ -20,7 +21,23 @@ const NOT_HTTP_TOKEN_CODE_POINT = /[^!#$%&'*+\-.^_`|~A-Za-z0-9]/g;
 const NOT_HTTP_QUOTED_STRING_CODE_POINT = /[^\t\u0020-~\u0080-\u00FF]/g;
 
 const END_BEGINNING_WHITESPACE = /[^\r\n\t ]|$/;
-const START_ENDING_WHITESPACE = /[\r\n\t ]*$/;
+
+// Index at which the run of trailing HTTP whitespace begins, or str.length
+// when there is none. Equivalent to the result of searching for
+// /[\r\n\t ]*$/, but a backward scan avoids the quadratic backtracking that an
+// unanchored search performs on a long internal whitespace run.
+function startOfTrailingWhitespace(str) {
+  let i = str.length;
+  while (i > 0) {
+    const c = StringPrototypeCharCodeAt(str, i - 1);
+    if (c === 0x09 || c === 0x0A || c === 0x0D || c === 0x20) {
+      i--;
+    } else {
+      break;
+    }
+  }
+  return i;
+}
 
 function toASCIILower(str) {
   // eslint-disable-next-line no-control-regex
@@ -68,7 +85,7 @@ function parseTypeAndSubtype(str) {
   const trimmedSubtype = StringPrototypeSlice(
     rawSubtype,
     0,
-    SafeStringPrototypeSearch(rawSubtype, START_ENDING_WHITESPACE));
+    startOfTrailingWhitespace(rawSubtype));
   const invalidSubtypeIndex = SafeStringPrototypeSearch(trimmedSubtype,
                                                         NOT_HTTP_TOKEN_CODE_POINT);
   if (trimmedSubtype === '' || invalidSubtypeIndex !== -1) {
@@ -225,9 +242,8 @@ class MIMEParams {
     const paramsMap = this.#data;
     let position = 0;
     const str = this.#string;
-    const endOfSource = SafeStringPrototypeSearch(
+    const endOfSource = startOfTrailingWhitespace(
       StringPrototypeSlice(str, position),
-      START_ENDING_WHITESPACE,
     ) + position;
     while (position < endOfSource) {
       // Skip any whitespace before parameter
@@ -291,7 +307,7 @@ class MIMEParams {
         const trimmedValue = StringPrototypeSlice(
           rawValue,
           0,
-          SafeStringPrototypeSearch(rawValue, START_ENDING_WHITESPACE),
+          startOfTrailingWhitespace(rawValue),
         );
         // Ignore parameters without values
         if (trimmedValue === '') continue;