diff --git a/library/core/benches/ascii.rs b/library/core/benches/ascii.rs
index bc59c378609f0..64938745a4a16 100644
--- a/library/core/benches/ascii.rs
+++ b/library/core/benches/ascii.rs
@@ -66,6 +66,8 @@ macro_rules! benches {
 use test::black_box;
 use test::Bencher;
 
+const ASCII_CASE_MASK: u8 = 0b0010_0000;
+
 benches! {
     fn case00_alloc_only(_bytes: &mut [u8]) {}
 
@@ -204,7 +206,7 @@ benches! {
             }
         }
         for byte in bytes {
-            *byte &= !((is_ascii_lowercase(*byte) as u8) << 5)
+            *byte &= !((is_ascii_lowercase(*byte) as u8) * ASCII_CASE_MASK)
         }
     }
 
@@ -216,7 +218,7 @@ benches! {
             }
         }
         for byte in bytes {
-            *byte -= (is_ascii_lowercase(*byte) as u8) << 5
+            *byte -= (is_ascii_lowercase(*byte) as u8) * ASCII_CASE_MASK
         }
     }
 
diff --git a/library/core/benches/char/methods.rs b/library/core/benches/char/methods.rs
index a9a08a4d76200..de4b63030fa7c 100644
--- a/library/core/benches/char/methods.rs
+++ b/library/core/benches/char/methods.rs
@@ -35,3 +35,13 @@ fn bench_to_digit_radix_var(b: &mut Bencher) {
             .min()
     })
 }
+
+#[bench]
+fn bench_to_ascii_uppercase(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_ascii_uppercase()).min())
+}
+
+#[bench]
+fn bench_to_ascii_lowercase(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_ascii_lowercase()).min())
+}
diff --git a/library/core/src/char/methods.rs b/library/core/src/char/methods.rs
index 2baea7842a796..3ddf0e638946c 100644
--- a/library/core/src/char/methods.rs
+++ b/library/core/src/char/methods.rs
@@ -1090,7 +1090,11 @@ impl char {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_uppercase(&self) -> char {
-        if self.is_ascii() { (*self as u8).to_ascii_uppercase() as char } else { *self }
+        if self.is_ascii_lowercase() {
+            (*self as u8).ascii_change_case_unchecked() as char
+        } else {
+            *self
+        }
     }
 
     /// Makes a copy of the value in its ASCII lower case equivalent.
@@ -1118,7 +1122,11 @@ impl char {
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     #[inline]
     pub fn to_ascii_lowercase(&self) -> char {
-        if self.is_ascii() { (*self as u8).to_ascii_lowercase() as char } else { *self }
+        if self.is_ascii_uppercase() {
+            (*self as u8).ascii_change_case_unchecked() as char
+        } else {
+            *self
+        }
     }
 
     /// Checks that two values are an ASCII case-insensitive match.
diff --git a/library/core/src/num/mod.rs b/library/core/src/num/mod.rs
index 6bdfa18fa434c..c13f000a73615 100644
--- a/library/core/src/num/mod.rs
+++ b/library/core/src/num/mod.rs
@@ -152,6 +152,9 @@ impl isize {
      usize_isize_to_xe_bytes_doc!(), usize_isize_from_xe_bytes_doc!() }
 }
 
+/// If 6th bit set ascii is upper case.
+const ASCII_CASE_MASK: u8 = 0b0010_0000;
+
 #[lang = "u8"]
 impl u8 {
     uint_impl! { u8, u8, 8, 255, 2, "0x82", "0xa", "0x12", "0x12", "0x48", "[0x12]",
@@ -195,7 +198,7 @@ impl u8 {
     #[inline]
     pub fn to_ascii_uppercase(&self) -> u8 {
         // Unset the fifth bit if this is a lowercase letter
-        *self & !((self.is_ascii_lowercase() as u8) << 5)
+        *self & !((self.is_ascii_lowercase() as u8) * ASCII_CASE_MASK)
     }
 
     /// Makes a copy of the value in its ASCII lower case equivalent.
@@ -218,7 +221,13 @@ impl u8 {
     #[inline]
     pub fn to_ascii_lowercase(&self) -> u8 {
         // Set the fifth bit if this is an uppercase letter
-        *self | ((self.is_ascii_uppercase() as u8) << 5)
+        *self | (self.is_ascii_uppercase() as u8 * ASCII_CASE_MASK)
+    }
+
+    /// Assumes self is ascii
+    #[inline]
+    pub(crate) fn ascii_change_case_unchecked(&self) -> u8 {
+        *self ^ ASCII_CASE_MASK
     }
 
     /// Checks that two values are an ASCII case-insensitive match.