diff --git a/.github/workflows/nix.yml b/.github/workflows/nix.yml
index 5f9d424..af19625 100644
--- a/.github/workflows/nix.yml
+++ b/.github/workflows/nix.yml
@@ -102,7 +102,7 @@ jobs:
           auto-push: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
           alert-threshold: '110%'
           fail-on-alert: true
-          fail-threshold: '120%'
+          fail-threshold: '130%'
           comment-on-alert: true
           comment-always: ${{ github.event_name == 'pull_request' }}
           benchmark-data-dir-path: dev/bench
diff --git a/src/decoder/complete.rs b/src/decoder/complete.rs
index 72eed34..67cc61d 100644
--- a/src/decoder/complete.rs
+++ b/src/decoder/complete.rs
@@ -31,7 +31,7 @@ pub(crate) type Table = [Entry; 256];
 
 #[inline(always)]
 pub(crate) fn decode_helper<'a>(table: &Table, src: &'a [u8]) -> Cow<'a, str> {
-    if src.is_ascii() {
+    if crate::is_ascii(src) {
         let s = unsafe { std::str::from_utf8_unchecked(src) };
         return s.into();
     }
diff --git a/src/decoder/incomplete.rs b/src/decoder/incomplete.rs
index adbb8e7..f0b6bbd 100644
--- a/src/decoder/incomplete.rs
+++ b/src/decoder/incomplete.rs
@@ -60,7 +60,7 @@ pub(crate) fn decode_helper<'a>(
     fallback: Option<char>,
 ) -> Result<Cow<'a, str>, DecodeError> {
     let fallback: Option<Entry> = fallback.map(Entry::from_char);
-    if bytes.is_ascii() {
+    if crate::is_ascii(bytes) {
         let s = unsafe { std::str::from_utf8_unchecked(bytes) };
         return Ok(s.into());
     }
diff --git a/src/encoder.rs b/src/encoder.rs
index a41d340..44809f3 100644
--- a/src/encoder.rs
+++ b/src/encoder.rs
@@ -12,7 +12,7 @@ pub trait Encoder {
         fallback: Option<u8>,
     ) -> Result<Cow<'a, [u8]>, EncodeError> {
         let mut src = s.as_bytes();
-        if s.is_ascii() {
+        if crate::is_ascii_str(s) {
             return Ok(src.into());
         }
         let len = s.chars().count();
diff --git a/src/lib.rs b/src/lib.rs
index 0997cb8..01ea3fa 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,7 +5,9 @@ use thiserror::Error;
 pub mod code_pages;
 pub(crate) mod decoder;
 mod encoder;
+mod simd;
 pub(crate) use encoder::Encoder;
+pub(crate) use simd::{is_ascii, is_ascii_str};
 
 #[derive(Error, Debug)]
 #[error("Character in UTF-8 string has no mapping defined in code page")]
diff --git a/src/simd.rs b/src/simd.rs
new file mode 100644
index 0000000..3b781c1
--- /dev/null
+++ b/src/simd.rs
@@ -0,0 +1,51 @@
+//! SIMD-optimized ASCII detection.
+//!
+//! Works around LLVM codegen issues with `-C target-cpu=native` on AVX512 CPUs.
+
+/// Check if all bytes are ASCII (< 128).
+#[inline]
+pub fn is_ascii(bytes: &[u8]) -> bool {
+    #[cfg(target_arch = "x86_64")]
+    if is_x86_feature_detected!("avx512bw") {
+        return unsafe { is_ascii_avx512(bytes) };
+    }
+    bytes.is_ascii()
+}
+
+/// Check if all characters in the string are ASCII.
+#[inline]
+pub fn is_ascii_str(s: &str) -> bool {
+    is_ascii(s.as_bytes())
+}
+
+#[cfg(target_arch = "x86_64")]
+#[target_feature(enable = "avx512bw")]
+/// # Safety
+/// Caller must ensure AVX512BW is available (via `is_x86_feature_detected!`).
+unsafe fn is_ascii_avx512(bytes: &[u8]) -> bool {
+    use core::arch::x86_64::*;
+
+    let ptr = bytes.as_ptr();
+    let len = bytes.len();
+    let mut i = 0;
+
+    // Process 64-byte chunks
+    // SAFETY: Loop condition ensures i+64 <= len
+    while i + 64 <= len {
+        if _mm512_movepi8_mask(_mm512_loadu_si512(ptr.add(i).cast())) != 0 {
+            return false;
+        }
+        i += 64;
+    }
+
+    // Tail: masked load for remaining bytes
+    if i < len {
+        // SAFETY: Mask has only `len - i` bits set, so only valid bytes are loaded
+        let mask = (1u64 << (len - i)) - 1;
+        if _mm512_movepi8_mask(_mm512_maskz_loadu_epi8(mask, ptr.add(i).cast())) != 0 {
+            return false;
+        }
+    }
+
+    true
+}