diff --git a/src/strings/strings.go b/src/strings/strings.go
index a7941fbb90..73bba9278c 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -542,7 +542,33 @@ func Repeat(s string, count int) string {
 }
 
 // ToUpper returns a copy of the string s with all Unicode letters mapped to their upper case.
-func ToUpper(s string) string { return Map(unicode.ToUpper, s) }
+func ToUpper(s string) string {
+	isASCII, hasLower := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasLower = hasLower || (c >= 'a' && c <= 'z')
+	}
+
+	if isASCII { // optimize for ASCII-only strings.
+		if !hasLower {
+			return s
+		}
+		b := make([]byte, len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if c >= 'a' && c <= 'z' {
+				c -= 'a' - 'A'
+			}
+			b[i] = c
+		}
+		return string(b)
+	}
+	return Map(unicode.ToUpper, s)
+}
 
 // ToLower returns a copy of the string s with all Unicode letters mapped to their lower case.
 func ToLower(s string) string { return Map(unicode.ToLower, s) }
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index 869be9c477..b185e7eec8 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -518,9 +518,12 @@ func runStringTests(t *testing.T, f func(string) string, funcName string, testCa
 
 var upperTests = []StringTest{
 	{"", ""},
+	{"ONLYUPPER", "ONLYUPPER"},
 	{"abc", "ABC"},
 	{"AbC123", "ABC123"},
 	{"azAZ09_", "AZAZ09_"},
+	{"longStrinGwitHmixofsmaLLandcAps", "LONGSTRINGWITHMIXOFSMALLANDCAPS"},
+	{"long\u0250string\u0250with\u0250nonascii\u2C6Fchars", "LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS"},
 	{"\u0250\u0250\u0250\u0250\u0250", "\u2C6F\u2C6F\u2C6F\u2C6F\u2C6F"}, // grows one byte per char
 }
 
@@ -648,6 +651,19 @@ 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 {
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToUpper(tc.in)
+				if actual != tc.out {
+					b.Errorf("ToUpper(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
 func BenchmarkMapNoChanges(b *testing.B) {
 	identity := func(r rune) rune {
 		return r