diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-11-02-05-27-54.gh-issue-161155.xXLPBV.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-11-02-05-27-54.gh-issue-161155.xXLPBV.rst
new file mode 100644
index 00000000000000..ba188f96a58b30
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-11-02-05-27-54.gh-issue-161155.xXLPBV.rst
@@ -0,0 +1,4 @@
+Speed up :meth:`str.lower`, :meth:`str.upper` and :meth:`str.casefold` for
+ASCII strings, and :meth:`bytes.lower`, :meth:`bytes.upper` and their
+:class:`bytearray` counterparts, by converting a machine word at a time.
+Patch by Parker Loflin.
diff --git a/Objects/bytes_methods.c b/Objects/bytes_methods.c
index 414afeb7bb003c..2e2236c90cdadc 100644
--- a/Objects/bytes_methods.c
+++ b/Objects/bytes_methods.c
@@ -242,6 +242,57 @@ _Py_bytes_istitle(const char *cptr, Py_ssize_t len)
 }
 
 
+/* Return 0x20 in each byte of x that lies in first..first+25, else 0,
+   given 0x80 - first and 0x80 - first - 26 in every byte of ge and gt.
+   For a byte b < 0x80, b + (0x80 - first) has its high bit set iff
+   b >= first, and b + (0x80 - first - 26) iff b > first + 25; neither
+   sum carries.  "& ~x" drops bytes >= 0x80. */
+static inline Py_ALWAYS_INLINE size_t
+ascii_case_bits(size_t x, size_t ge, size_t gt)
+{
+    const size_t ones = (size_t)-1 / 0xff;  /* 0x01 in every byte */
+    size_t b = x & (0x7f * ones);
+    size_t ge_first = b + ge;
+    size_t gt_last = b + gt;
+    return (ge_first & ~gt_last & ~x & (0x80 * ones)) >> 2;
+}
+
+static inline Py_ALWAYS_INLINE void
+ascii_case_word(char *result, const char *cptr, Py_ssize_t i,
+                size_t ge, size_t gt)
+{
+    size_t x;
+    memcpy(&x, cptr + i, sizeof(x));
+    x ^= ascii_case_bits(x, ge, gt);
+    memcpy(result + i, &x, sizeof(x));
+}
+
+/* Requires len >= SIZEOF_SIZE_T.  The last word may overlap the one before
+   it; converting a byte twice is harmless.  Not inlined, so that short
+   strings skip its setup. */
+static Py_NO_INLINE void
+ascii_case_words(char *result, const char *cptr, Py_ssize_t len,
+                 unsigned char first)
+{
+    const size_t ones = (size_t)-1 / 0xff;
+    size_t ge = (0x80 - first) * ones, gt = (0x80 - first - 26) * ones;
+    Py_ssize_t i;
+#if defined(_MSC_VER) && !defined(__clang__)
+    /* MSVC only vectorizes loops that count by one; elsewhere such a loop
+       gets post-indexed accesses, slow on Apple cores.  Dividing unsigned
+       avoids an idiv in code optimized for size. */
+    Py_ssize_t n = (size_t)(len - 1) / SIZEOF_SIZE_T;
+    for (i = 0; i < n; i++) {
+        ascii_case_word(result, cptr, i * SIZEOF_SIZE_T, ge, gt);
+    }
+#else
+    for (i = 0; i < len - SIZEOF_SIZE_T; i += SIZEOF_SIZE_T) {
+        ascii_case_word(result, cptr, i, ge, gt);
+    }
+#endif
+    ascii_case_word(result, cptr, len - SIZEOF_SIZE_T, ge, gt);
+}
+
 PyDoc_STRVAR_shared(_Py_lower__doc__,
 "B.lower() -> copy of B\n\
 \n\
@@ -252,6 +303,10 @@ _Py_bytes_lower(char *result, const char *cptr, Py_ssize_t len)
 {
     Py_ssize_t i;
 
+    if (len >= SIZEOF_SIZE_T) {
+        ascii_case_words(result, cptr, len, 'A');
+        return;
+    }
     for (i = 0; i < len; i++) {
         result[i] = Py_TOLOWER((unsigned char) cptr[i]);
     }
@@ -268,6 +323,10 @@ _Py_bytes_upper(char *result, const char *cptr, Py_ssize_t len)
 {
     Py_ssize_t i;
 
+    if (len >= SIZEOF_SIZE_T) {
+        ascii_case_words(result, cptr, len, 'a');
+        return;
+    }
     for (i = 0; i < len; i++) {
         result[i] = Py_TOUPPER((unsigned char) cptr[i]);
     }