diff --git a/src/workerd/api/util.c++ b/src/workerd/api/util.c++
index e4753486909..b36e4df16c0 100644
--- a/src/workerd/api/util.c++
+++ b/src/workerd/api/util.c++
@@ -40,19 +40,61 @@ kj::ArrayPtr<const char> split(kj::ArrayPtr<const char>& text, char c) {
   return result;
 }
 
+constexpr uint64_t broadcast(uint8_t v) noexcept {
+  return 0x101010101010101ull * v;
+}
+
+// SWAR routine designed to convert ASCII uppercase letters to lowercase.
+// Let's process 8 bytes (64 bits) at a time using a single 64-bit int,
+// treating as 8 parallel 8-bit values.
+// PS: This will enable the use of auto-vectorization.
+constexpr void toLowerAscii(char* input, size_t length) noexcept {
+  constexpr const uint64_t broadcast_80 = broadcast(0x80);
+  constexpr const uint64_t broadcast_Ap = broadcast(128 - 'A');
+  constexpr const uint64_t broadcast_Zp = broadcast(128 - 'Z' - 1);
+  size_t i = 0;
+  for (; i + 7 < length; i += 8) {
+    uint64_t word{};
+    memcpy(&word, input + i, sizeof(word));
+    word ^= (((word + broadcast_Ap) ^ (word + broadcast_Zp)) & broadcast_80) >> 2;
+    memcpy(input + i, &word, sizeof(word));
+  }
+  if (i < length) {
+    uint64_t word{};
+    memcpy(&word, input + i, length - i);
+    word ^= (((word + broadcast_Ap) ^ (word + broadcast_Zp)) & broadcast_80) >> 2;
+    memcpy(input + i, &word, length - i);
+  }
+}
+
+constexpr void toUpperAscii(char* input, size_t length) noexcept {
+  constexpr const uint64_t broadcast_80 = broadcast(0x80);
+  constexpr const uint64_t broadcast_ap = broadcast(128 - 'a');
+  constexpr const uint64_t broadcast_zp = broadcast(128 - 'z' - 1);
+  size_t i = 0;
+  for (; i + 7 < length; i += 8) {
+    uint64_t word{};
+    memcpy(&word, input + i, sizeof(word));
+    word ^= (((word + broadcast_ap) ^ (word + broadcast_zp)) & broadcast_80) >> 2;
+    memcpy(input + i, &word, sizeof(word));
+  }
+  if (i < length) {
+    uint64_t word{};
+    memcpy(&word, input + i, length - i);
+    word ^= (((word + broadcast_ap) ^ (word + broadcast_zp)) & broadcast_80) >> 2;
+    memcpy(input + i, &word, length - i);
+  }
+}
+
 }  // namespace
 
 kj::String toLower(kj::String&& str) {
-  for (char& c: str) {
-    if ('A' <= c && c <= 'Z') c += 'a' - 'A';
-  }
+  toLowerAscii(str.begin(), str.size());
   return kj::mv(str);
 }
 
 kj::String toUpper(kj::String&& str) {
-  for (char& c: str) {
-    if ('a' <= c && c <= 'z') c -= 'a' - 'A';
-  }
+  toUpperAscii(str.begin(), str.size());
   return kj::mv(str);
 }