diff --git a/src/strings/strings.go b/src/strings/strings.go
index 4ab78c326bc..7921a20e8ab 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -568,14 +568,24 @@ func ToUpper(s string) string {
 		if !hasLower {
 			return s
 		}
-		var b Builder
+		var (
+			b   Builder
+			pos int
+		)
 		b.Grow(len(s))
 		for i := 0; i < len(s); i++ {
 			c := s[i]
 			if 'a' <= c && c <= 'z' {
 				c -= 'a' - 'A'
+				if pos < i {
+					b.WriteString(s[pos:i])
+				}
+				b.WriteByte(c)
+				pos = i + 1
 			}
-			b.WriteByte(c)
+		}
+		if pos < len(s) {
+			b.WriteString(s[pos:])
 		}
 		return b.String()
 	}
@@ -598,14 +608,24 @@ func ToLower(s string) string {
 		if !hasUpper {
 			return s
 		}
-		var b Builder
+		var (
+			b   Builder
+			pos int
+		)
 		b.Grow(len(s))
 		for i := 0; i < len(s); i++ {
 			c := s[i]
 			if 'A' <= c && c <= 'Z' {
 				c += 'a' - 'A'
+				if pos < i {
+					b.WriteString(s[pos:i])
+				}
+				b.WriteByte(c)
+				pos = i + 1
 			}
-			b.WriteByte(c)
+		}
+		if pos < len(s) {
+			b.WriteString(s[pos:])
 		}
 		return b.String()
 	}
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index d6153aa2263..6d394f47bed 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -549,6 +549,7 @@ var upperTests = []StringTest{
 	{"AbC123", "ABC123"},
 	{"azAZ09_", "AZAZ09_"},
 	{"longStrinGwitHmixofsmaLLandcAps", "LONGSTRINGWITHMIXOFSMALLANDCAPS"},
+	{"RENAN BASTOS 93 AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO", "RENAN BASTOS 93 AOSDAJDJAIDJAIDAJIAIDSJJAIDIJADSJIADJIOOKKO"},
 	{"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
 	{"a\u0080\U0010FFFF", "A\u0080\U0010FFFF"},                           // test utf8.RuneSelf and utf8.MaxRune
@@ -560,6 +561,7 @@ var lowerTests = []StringTest{
 	{"AbC123", "abc123"},
 	{"azAZ09_", "azaz09_"},
 	{"longStrinGwitHmixofsmaLLandcAps", "longstringwithmixofsmallandcaps"},
+	{"renan bastos 93 AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO", "renan bastos 93 aosdajdjaidjaidajiaidsjjaidijadsjiadjiookko"},
 	{"LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS", "long\u0250string\u0250with\u0250nonascii\u0250chars"},
 	{"\u2C6D\u2C6D\u2C6D\u2C6D\u2C6D", "\u0251\u0251\u0251\u0251\u0251"}, // shrinks one byte per char
 	{"A\u0080\U0010FFFF", "a\u0080\U0010FFFF"},                           // test utf8.RuneSelf and utf8.MaxRune