diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index bdd55fca4a7..22aeded5e13 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -521,11 +521,66 @@ func Repeat(b []byte, count int) []byte {
 	return nb
 }
 
-// ToUpper treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters within it mapped to their upper case.
-func ToUpper(s []byte) []byte { return Map(unicode.ToUpper, s) }
+// ToUpper returns a copy of the byte slice s with all Unicode letters mapped to
+// their upper case.
+func ToUpper(s []byte) []byte {
+	isASCII, hasLower := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasLower = hasLower || ('a' <= c && c <= 'z')
+	}
 
-// ToLower treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their lower case.
-func ToLower(s []byte) []byte { return Map(unicode.ToLower, s) }
+	if isASCII { // optimize for ASCII-only byte slices.
+		if !hasLower {
+			// Just return a copy.
+			return append([]byte(""), s...)
+		}
+		b := make([]byte, len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'a' <= c && c <= 'z' {
+				c -= 'a' - 'A'
+			}
+			b[i] = c
+		}
+		return b
+	}
+	return Map(unicode.ToUpper, s)
+}
+
+// ToLower returns a copy of the byte slice s with all Unicode letters mapped to
+// their lower case.
+func ToLower(s []byte) []byte {
+	isASCII, hasUpper := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
+	}
+
+	if isASCII { // optimize for ASCII-only byte slices.
+		if !hasUpper {
+			return append([]byte(""), s...)
+		}
+		b := make([]byte, len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'A' <= c && c <= 'Z' {
+				c += 'a' - 'A'
+			}
+			b[i] = c
+		}
+		return b
+	}
+	return Map(unicode.ToLower, s)
+}
 
 // ToTitle treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their title case.
 func ToTitle(s []byte) []byte { return Map(unicode.ToTitle, s) }
diff --git a/src/bytes/bytes_test.go b/src/bytes/bytes_test.go
index d760d4b52ab..340810facf4 100644
--- a/src/bytes/bytes_test.go
+++ b/src/bytes/bytes_test.go
@@ -891,10 +891,14 @@ type StringTest struct {
 
 var upperTests = []StringTest{
 	{"", []byte("")},
+	{"ONLYUPPER", []byte("ONLYUPPER")},
 	{"abc", []byte("ABC")},
 	{"AbC123", []byte("ABC123")},
 	{"azAZ09_", []byte("AZAZ09_")},
+	{"longStrinGwitHmixofsmaLLandcAps", []byte("LONGSTRINGWITHMIXOFSMALLANDCAPS")},
+	{"long\u0250string\u0250with\u0250nonascii\u2C6Fchars", []byte("LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS")},
 	{"\u0250\u0250\u0250\u0250\u0250", []byte("\u2C6F\u2C6F\u2C6F\u2C6F\u2C6F")}, // grows one byte per char
+	{"a\u0080\U0010FFFF", []byte("A\u0080\U0010FFFF")},                           // test utf8.RuneSelf and utf8.MaxRune
 }
 
 var lowerTests = []StringTest{
@@ -902,7 +906,10 @@ var lowerTests = []StringTest{
 	{"abc", []byte("abc")},
 	{"AbC123", []byte("abc123")},
 	{"azAZ09_", []byte("azaz09_")},
+	{"longStrinGwitHmixofsmaLLandcAps", []byte("longstringwithmixofsmallandcaps")},
+	{"LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS", []byte("long\u0250string\u0250with\u0250nonascii\u0250chars")},
 	{"\u2C6D\u2C6D\u2C6D\u2C6D\u2C6D", []byte("\u0251\u0251\u0251\u0251\u0251")}, // shrinks one byte per char
+	{"A\u0080\U0010FFFF", []byte("a\u0080\U0010FFFF")},                           // test utf8.RuneSelf and utf8.MaxRune
 }
 
 const space = "\t\v\r\f\n\u0085\u00a0\u2000\u3000"
@@ -1029,6 +1036,34 @@ func TestToUpper(t *testing.T) { runStringTests(t, ToUpper, "ToUpper", upperTest
 
 func TestToLower(t *testing.T) { runStringTests(t, ToLower, "ToLower", lowerTests) }
 
+func BenchmarkToUpper(b *testing.B) {
+	for _, tc := range upperTests {
+		tin := []byte(tc.in)
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToUpper(tin)
+				if !Equal(actual, tc.out) {
+					b.Errorf("ToUpper(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkToLower(b *testing.B) {
+	for _, tc := range lowerTests {
+		tin := []byte(tc.in)
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToLower(tin)
+				if !Equal(actual, tc.out) {
+					b.Errorf("ToLower(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
 func TestTrimSpace(t *testing.T) { runStringTests(t, TrimSpace, "TrimSpace", trimSpaceTests) }
 
 type RepeatTest struct {